diff --git a/src/editor/caret.js b/src/editor/caret.js
new file mode 100644
index 0000000000..3b803f35c3
--- /dev/null
+++ b/src/editor/caret.js
@@ -0,0 +1,78 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export function getCaretPosition(editor) {
+    const sel = document.getSelection();
+    const atNodeEnd = sel.focusOffset === sel.focusNode.textContent.length;
+    let position = sel.focusOffset;
+    let node = sel.focusNode;
+
+    // when deleting the last character of a node,
+    // the caret gets reported as being after the focusOffset-th node,
+    // with the focusNode being the editor
+    if (node === editor) {
+        let position = 0;
+        for (let i = 0; i < sel.focusOffset; ++i) {
+            position += editor.childNodes[i].textContent.length;
+        }
+        return {position, atNodeEnd: false};
+    }
+
+    // first make sure we're at the level of a direct child of editor
+    if (node.parentElement !== editor) {
+        // include all preceding siblings of the non-direct editor children
+        while (node.previousSibling) {
+            node = node.previousSibling;
+            position += node.textContent.length;
+        }
+        // then move up
+        // I guess technically there could be preceding text nodes in the parents here as well,
+        // but we're assuming there are no mixed text and element nodes
+        while (node.parentElement !== editor) {
+            node = node.parentElement;
+        }
+    }
+    // now include the text length of all preceding direct editor children
+    while (node.previousSibling) {
+        node = node.previousSibling;
+        position += node.textContent.length;
+    }
+    {
+        const {focusOffset, focusNode} = sel;
+        console.log("selection", {focusOffset, focusNode, position, atNodeEnd});
+    }
+    return {position, atNodeEnd};
+}
+
+export function setCaretPosition(editor, caretPosition) {
+    if (caretPosition) {
+        let focusNode = editor.childNodes[caretPosition.index];
+        if (!focusNode) {
+            focusNode = editor;
+        } else {
+            // make sure we have a text node
+            if (focusNode.nodeType === Node.ELEMENT_NODE) {
+                focusNode = focusNode.childNodes[0];
+            }
+        }
+        const sel = document.getSelection();
+        sel.removeAllRanges();
+        const range = document.createRange();
+        range.setStart(focusNode, caretPosition.offset);
+        range.collapse(true);
+        sel.addRange(range);
+    }
+}
diff --git a/src/editor/diff.js b/src/editor/diff.js
new file mode 100644
index 0000000000..6dc8b746e4
--- /dev/null
+++ b/src/editor/diff.js
@@ -0,0 +1,78 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+function firstDiff(a, b) {
+    const compareLen = Math.min(a.length, b.length);
+    for (let i = 0; i < compareLen; ++i) {
+        if (a[i] !== b[i]) {
+            return i;
+        }
+    }
+    return compareLen;
+}
+
+function lastDiff(a, b) {
+    const compareLen = Math.min(a.length, b.length);
+    for (let i = 0; i < compareLen; ++i) {
+        if (a[a.length - i] !== b[b.length - i]) {
+            return i;
+        }
+    }
+    return compareLen;
+}
+
+function diffStringsAtEnd(oldStr, newStr) {
+    const len = Math.min(oldStr.length, newStr.length);
+    const startInCommon = oldStr.substr(0, len) === newStr.substr(0, len);
+    if (startInCommon && oldStr.length > newStr.length) {
+        return {removed: oldStr.substr(len), at: len};
+    } else if (startInCommon && oldStr.length < newStr.length) {
+        return {added: newStr.substr(len), at: len};
+    } else {
+        const commonStartLen = firstDiff(oldStr, newStr);
+        return {
+            removed: oldStr.substr(commonStartLen),
+            added: newStr.substr(commonStartLen),
+            at: commonStartLen,
+        };
+    }
+}
+
+export function diffDeletion(oldStr, newStr) {
+    if (oldStr === newStr) {
+        return {};
+    }
+    const firstDiffIdx = firstDiff(oldStr, newStr);
+    const lastDiffIdx = oldStr.length - lastDiff(oldStr, newStr) + 1;
+    return {at: firstDiffIdx, removed: oldStr.substring(firstDiffIdx, lastDiffIdx)};
+}
+
+export function diffInsertion(oldStr, newStr) {
+    const diff = diffDeletion(newStr, oldStr);
+    if (diff.removed) {
+        return {at: diff.at, added: diff.removed};
+    } else {
+        return diff;
+    }
+}
+
+export function diffAtCaret(oldValue, newValue, caretPosition) {
+    const diffLen = newValue.length - oldValue.length;
+    const caretPositionBeforeInput = caretPosition - diffLen;
+    const oldValueBeforeCaret = oldValue.substr(0, caretPositionBeforeInput);
+    const newValueBeforeCaret = newValue.substr(0, caretPosition);
+    return diffStringsAtEnd(oldValueBeforeCaret, newValueBeforeCaret);
+}
diff --git a/src/editor/model.js b/src/editor/model.js
new file mode 100644
index 0000000000..ffd2e17c01
--- /dev/null
+++ b/src/editor/model.js
@@ -0,0 +1,169 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {PlainPart, RoomPillPart, UserPillPart} from "./parts";
+import {diffAtCaret, diffDeletion} from "./diff";
+
+export default class EditorModel {
+    constructor(parts = []) {
+        this._parts = parts;
+        this.actions = null;
+        this._previousValue = parts.reduce((text, p) => text + p.text, "");
+    }
+
+    _insertPart(index, part) {
+        this._parts.splice(index, 0, part);
+    }
+
+    _removePart(index) {
+        this._parts.splice(index, 1);
+    }
+
+    _replacePart(index, part) {
+        this._parts.splice(index, 1, part);
+    }
+
+    get parts() {
+        return this._parts;
+    }
+
+    _diff(newValue, inputType, caret) {
+        if (inputType === "deleteByDrag") {
+            return diffDeletion(this._previousValue, newValue);
+        } else {
+            return diffAtCaret(this._previousValue, newValue, caret.position);
+        }
+    }
+
+    update(newValue, inputType, caret) {
+        const diff = this._diff(newValue, inputType, caret);
+        const position = this._positionForOffset(diff.at, caret.atNodeEnd);
+        console.log("update at", {position, diff});
+        if (diff.removed) {
+            this._removeText(position, diff.removed.length);
+        }
+        if (diff.added) {
+            this._addText(position, diff.added);
+        }
+        this._mergeAdjacentParts();
+        this._previousValue = newValue;
+        const caretOffset = diff.at + (diff.added ? diff.added.length : 0);
+        return this._positionForOffset(caretOffset, true);
+    }
+
+    _mergeAdjacentParts(docPos) {
+        let prevPart = this._parts[0];
+        for (let i = 1; i < this._parts.length; ++i) {
+            let part = this._parts[i];
+            const isEmpty = !part.text.length;
+            const isMerged = !isEmpty && prevPart.merge(part);
+            if (isEmpty || isMerged) {
+                // remove empty or merged part
+                part = prevPart;
+                this._removePart(i);
+                //repeat this index, as it's removed now
+                --i;
+            }
+            prevPart = part;
+        }
+    }
+
+    _removeText(pos, len) {
+        let {index, offset} = pos;
+        while (len !== 0) {
+            // part might be undefined here
+            let part = this._parts[index];
+            const amount = Math.min(len, part.text.length - offset);
+            const replaceWith = part.remove(offset, amount);
+            if (typeof replaceWith === "string") {
+                this._replacePart(index, new PlainPart(replaceWith));
+            }
+            part = this._parts[index];
+            // remove empty part
+            if (!part.text.length) {
+                this._removePart(index);
+            } else {
+                index += 1;
+            }
+            len -= amount;
+            offset = 0;
+        }
+    }
+
+    _addText(pos, str, actions) {
+        let {index, offset} = pos;
+        const part = this._parts[index];
+        if (part) {
+            if (part.insertAll(offset, str)) {
+                str = null;
+            } else {
+                // console.log("splitting", offset, [part.text]);
+                const splitPart = part.split(offset);
+                // console.log("splitted", [part.text, splitPart.text]);
+                index += 1;
+                this._insertPart(index, splitPart);
+            }
+        }
+        while (str) {
+            let newPart;
+            switch (str[0]) {
+                case "#":
+                    newPart = new RoomPillPart();
+                    break;
+                case "@":
+                    newPart = new UserPillPart();
+                    break;
+                default:
+                    newPart = new PlainPart();
+            }
+            str = newPart.appendUntilRejected(str);
+            this._insertPart(index, newPart);
+            index += 1;
+        }
+    }
+
+    _positionForOffset(totalOffset, atPartEnd) {
+        let currentOffset = 0;
+        const index = this._parts.findIndex(part => {
+            const partLen = part.text.length;
+            if (
+                (atPartEnd && (currentOffset + partLen) >= totalOffset) ||
+                (!atPartEnd && (currentOffset + partLen) > totalOffset)
+            ) {
+                return true;
+            }
+            currentOffset += partLen;
+            return false;
+        });
+
+        return new DocumentPosition(index, totalOffset - currentOffset);
+    }
+}
+
+class DocumentPosition {
+    constructor(index, offset) {
+        this._index = index;
+        this._offset = offset;
+    }
+
+    get index() {
+        return this._index;
+    }
+
+    get offset() {
+        return this._offset;
+    }
+}
diff --git a/src/editor/parts.js b/src/editor/parts.js
new file mode 100644
index 0000000000..be5326d98f
--- /dev/null
+++ b/src/editor/parts.js
@@ -0,0 +1,174 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+class BasePart {
+    constructor(text = "") {
+        this._text = text;
+    }
+
+    acceptsInsertion(chr) {
+        return true;
+    }
+
+    acceptsRemoval(position, chr) {
+        return true;
+    }
+
+    merge(part) {
+        return false;
+    }
+
+    split(offset) {
+        const splitText = this.text.substr(offset);
+        this._text = this.text.substr(0, offset);
+        return new PlainPart(splitText);
+    }
+
+    // removes len chars, or returns the plain text this part should be replaced with
+    // if the part would become invalid if it removed everything.
+
+    // TODO: this should probably return the Part and caret position within this should be replaced with
+    remove(offset, len) {
+        // validate
+        const strWithRemoval = this.text.substr(0, offset) + this.text.substr(offset + len);
+        for(let i = offset; i < (len + offset); ++i) {
+            const chr = this.text.charAt(i);
+            if (!this.acceptsRemoval(i, chr)) {
+                return strWithRemoval;
+            }
+        }
+        this._text = strWithRemoval;
+    }
+
+    // append str, returns the remaining string if a character was rejected.
+    appendUntilRejected(str) {
+        for(let i = 0; i < str.length; ++i) {
+            const chr = str.charAt(i);
+            if (!this.acceptsInsertion(chr)) {
+                this._text = this._text + str.substr(0, i);
+                return str.substr(i);
+            }
+        }
+        this._text = this._text + str;
+    }
+
+    // inserts str at offset if all the characters in str were accepted, otherwise don't do anything
+    // return whether the str was accepted or not.
+    insertAll(offset, str) {
+        for(let i = 0; i < str.length; ++i) {
+            const chr = str.charAt(i);
+            if (!this.acceptsInsertion(chr)) {
+                return false;
+            }
+        }
+        const beforeInsert = this._text.substr(0, offset);
+        const afterInsert = this._text.substr(offset);
+        this._text = beforeInsert + str + afterInsert;
+        return true;
+    }
+
+
+    trim(len) {
+        const remaining = this._text.substr(len);
+        this._text = this._text.substr(0, len);
+        return remaining;
+    }
+
+    get text() {
+        return this._text;
+    }
+}
+
+export class PlainPart extends BasePart {
+    acceptsInsertion(chr) {
+        return chr !== "@" && chr !== "#";
+    }
+
+    toDOMNode() {
+        return document.createTextNode(this.text);
+    }
+
+    merge(part) {
+        if (part.type === this.type) {
+            this._text = this.text + part.text;
+            return true;
+        }
+        return false;
+    }
+
+    get type() {
+        return "plain";
+    }
+
+    updateDOMNode(node) {
+        if (node.textContent !== this.text) {
+            // console.log("changing plain text from", node.textContent, "to", this.text);
+            node.textContent = this.text;
+        }
+    }
+
+    canUpdateDOMNode(node) {
+        return node.nodeType === Node.TEXT_NODE;
+    }
+}
+
+class PillPart extends BasePart {
+    acceptsInsertion(chr) {
+        return chr !== " ";
+    }
+
+    acceptsRemoval(position, chr) {
+        return position !== 0;  //if you remove initial # or @, pill should become plain
+    }
+
+    toDOMNode() {
+        const container = document.createElement("span");
+        container.className = this.type;
+        container.appendChild(document.createTextNode(this.text));
+        return container;
+    }
+
+    updateDOMNode(node) {
+        const textNode = node.childNodes[0];
+        if (textNode.textContent !== this.text) {
+            // console.log("changing pill text from", textNode.textContent, "to", this.text);
+            textNode.textContent = this.text;
+        }
+        if (node.className !== this.type) {
+            // console.log("turning", node.className, "into", this.type);
+            node.className = this.type;
+        }
+    }
+
+    canUpdateDOMNode(node) {
+        return node.nodeType === Node.ELEMENT_NODE &&
+               node.nodeName === "SPAN" &&
+               node.childNodes.length === 1 &&
+               node.childNodes[0].nodeType === Node.TEXT_NODE;
+    }
+}
+
+export class RoomPillPart extends PillPart {
+    get type() {
+        return "room-pill";
+    }
+}
+
+export class UserPillPart extends PillPart {
+    get type() {
+        return "user-pill";
+    }
+}