The purpose of these studies is to characterize children's conception of reversal and its relation to a reference state. A reversal is the move from one state to some prior state of affairs. For example, shoes that have been TIED can be UNTIED, parcels WRAPPED then UNWRAPPED, and dishes COVERED then UNCOVERED. The present studies were designed to find out how children (aged 1;0 to 5;0) describe reversals of action that restore objects to a prior, less constrained, state. In English, the prefix un- offers the most productive device for this, but, initially, children rely on a verb like open, on general purpose undo, and on particles like out and off, As they acquire un-, English-speaking children must learn that this prefix applies primarily to verbs for change-of-state, often for enclosing, covering and attaching. In German, there is no reversal prefix, but there are productive particles. German-speaking children also begin with a verb like open and then turn to verb particles on a course similar to that in English to express reversals.