diff --git a/source/reducers/reduceSteps.js b/source/reducers/reduceSteps.js
new file mode 100644
index 000000000..2182d62bb
--- /dev/null
+++ b/source/reducers/reduceSteps.js
@@ -0,0 +1,146 @@
+import { path, head, reject, concat, without } from 'ramda'
+
+import { rules, collectDefaults, rulesFr } from 'Engine/rules'
+import {
+	getNextSteps,
+	collectMissingVariablesByTarget
+} from 'Engine/generateQuestions'
+import { analyseMany, parseAll } from 'Engine/traverse'
+
+export default (tracker, flatRules, answerSource) => (state, action) => {
+	state.flatRules = flatRules
+	// Optimization - don't parse on each analysis
+	if (!state.parsedRules) {
+		state.parsedRules = parseAll(flatRules)
+	}
+
+	// TODO
+	if (action.type == 'CHANGE_LANG') {
+		if (action.lang == 'fr') {
+			flatRules = rulesFr
+		} else flatRules = rules
+		return {
+			...state,
+			flatRules
+		}
+	}
+
+	if (
+		![
+			'SET_CONVERSATION_TARGETS',
+			'STEP_ACTION',
+			'USER_INPUT_UPDATE',
+			'START_CONVERSATION',
+			'SET_ACTIVE_TARGET_INPUT'
+		].includes(action.type)
+	)
+		return state
+
+	if (
+		path(['form', 'conversation', 'syncErrors'], state) ||
+		!answerSource(state)(state.activeTargetInput)
+	)
+		return state
+
+	let targetNames = reject(
+		name => state.activeTargetInput && state.activeTargetInput.includes(name)
+	)(state.targetNames)
+
+	// Most rules have default values
+	let rulesDefaults = collectDefaults(flatRules),
+		situationWithDefaults = assume(answerSource, rulesDefaults)
+
+	let analysis = analyseMany(state.parsedRules, targetNames)(
+		situationWithDefaults(state)
+	)
+
+	if (action.type === 'USER_INPUT_UPDATE') {
+		return { ...state, analysis, situationGate: situationWithDefaults(state) }
+	}
+
+	let nextStepsAnalysis = analyseMany(state.parsedRules, targetNames)(
+			answerSource(state)
+		),
+		missingVariablesByTarget = collectMissingVariablesByTarget(
+			nextStepsAnalysis.targets
+		),
+		nextSteps = getNextSteps(missingVariablesByTarget),
+		currentQuestion = head(nextSteps)
+
+	let newState = {
+		...state,
+		analysis,
+		situationGate: situationWithDefaults(state),
+		explainedVariable: null,
+		nextSteps,
+		currentQuestion,
+		foldedSteps:
+			action.type === 'SET_CONVERSATION_TARGETS' && action.reset
+				? []
+				: state.foldedSteps
+	}
+
+	if (['START_CONVERSATION', 'SET_ACTIVE_TARGET_INPUT'].includes(action.type))
+		return {
+			...newState,
+			missingVariablesByTarget: {
+				initial: missingVariablesByTarget,
+				current: missingVariablesByTarget
+			}
+		}
+
+	if (action.type == 'STEP_ACTION' && action.name == 'fold') {
+		tracker.push([
+			'trackEvent',
+			'answer:' + action.source,
+			action.step + ': ' + situationWithDefaults(state)(action.step)
+		])
+
+		if (!newState.currentQuestion) {
+			tracker.push([
+				'trackEvent',
+				'done',
+				'after' + length(newState.foldedSteps) + 'questions'
+			])
+		}
+		let foldedSteps = [...state.foldedSteps, state.currentQuestion]
+
+		return {
+			...newState,
+			foldedSteps,
+			missingVariablesByTarget: {
+				...state.missingVariablesByTarget,
+				current: missingVariablesByTarget
+			}
+		}
+	}
+	if (action.type == 'STEP_ACTION' && action.name == 'unfold') {
+		tracker.push(['trackEvent', 'unfold', action.step])
+
+		// We are possibly "refolding" a previously open question
+		let previous = state.currentQuestion,
+			// we fold it back into foldedSteps if it had been answered
+			answered = previous && answerSource(state)(previous) != undefined,
+			rawFoldedSteps = answered
+				? concat(state.foldedSteps, [previous])
+				: state.foldedSteps,
+			foldedSteps = without([action.step], rawFoldedSteps)
+
+		return {
+			...newState,
+			foldedSteps,
+			currentQuestion: action.step,
+			missingVariablesByTarget: {
+				...state.missingVariablesByTarget,
+				current: missingVariablesByTarget
+			}
+		}
+	}
+}
+
+// assume "wraps" a given situation function with one that overrides its values with
+// the given assumptions
+export let assume = (evaluator, assumptions) => state => name => {
+	let userInput = evaluator(state)(name)
+	return userInput != null ? userInput : assumptions[name]
+}
diff --git a/source/reducers/reducers.js b/source/reducers/reducers.js
new file mode 100644
index 000000000..349fe3474
--- /dev/null
+++ b/source/reducers/reducers.js
@@ -0,0 +1,90 @@
+import { combineReducers } from 'redux'
+import reduceReducers from 'reduce-reducers'
+import { reducer as formReducer, formValueSelector } from 'redux-form'
+import reduceSteps from './reduceSteps'
+import computeThemeColours from 'Components/themeColours'
+import { formatInputs } from 'Engine/rules'
+
+import ReactPiwik from 'Components/Tracker'
+
+import { popularTargetNames } from 'Components/TargetSelection'
+
+function themeColours(state = computeThemeColours(), { type, colour }) {
+	if (type == 'CHANGE_THEME_COLOUR') return computeThemeColours(colour)
+	else return state
+}
+
+function explainedVariable(state = null, { type, variableName = null }) {
+	switch (type) {
+		case 'EXPLAIN_VARIABLE':
+			return variableName
+		default:
+			return state
+	}
+}
+
+function currentExample(state = null, { type, situation, name }) {
+	switch (type) {
+		case 'SET_EXAMPLE':
+			return name != null ? { name, situation } : null
+		default:
+			return state
+	}
+}
+
+function conversationStarted(state = false, { type }) {
+	switch (type) {
+		case 'START_CONVERSATION':
+			return true
+		default:
+			return state
+	}
+}
+function activeTargetInput(state = null, { type, name }) {
+	switch (type) {
+		case 'SET_ACTIVE_TARGET_INPUT':
+			return name
+		default:
+			return state
+	}
+}
+
+export default initialRules =>
+	reduceReducers(
+		combineReducers({
+			sessionId: (id = Math.floor(Math.random() * 1000000000000) + '') => id,
+			//  this is handled by redux-form, pas touche !
+			form: formReducer,
+
+			/* Have forms been filled or ignored ?
+		false means the user is reconsidering its previous input */
+			foldedSteps: (steps = []) => steps,
+			currentQuestion: (state = null) => state,
+			nextSteps: (state = []) => state,
+			missingVariablesByTarget: (state = {}) => state,
+
+			parsedRules: (state = null) => state,
+			flatRules: (state = null) => state,
+			analysis: (state = null) => state,
+
+			targetNames: (state = popularTargetNames) => state,
+
+			situationGate: (state = name => null) => state,
+
+			iframe: (state = false) => state,
+
+			themeColours,
+
+			explainedVariable,
+
+			currentExample,
+			conversationStarted,
+			activeTargetInput
+		}),
+		// cross-cutting concerns because here `state` is the whole state tree
+		reduceSteps(
+			ReactPiwik,
+			initialRules,
+			formatInputs(initialRules, formValueSelector)
+		)
+	)