One frequently asked question is whether Redux "calls all reducers" when dispatching an action. Since there really is only one root reducer function, the default answer is "no, it does not". However, combineReducers has specific behavior that does work that way. In order to assemble the new state tree, combineReducers will call each slice reducer with its current slice of state and the current action, giving the slice reducer a chance to respond and update its slice of state if needed. So, in that sense, using combineReducers does "call all reducers", or at least all of the slice reducers it is wrapping.