This paper presents a critical reconstruction of the main Marxist debates about the idea of 'leaps forward' in historical development. There have been two important approaches: the so-called 'law of uneven and combined development', as developed by Leon Trotsky, George Novack and Ernest Mandel, and Jan Romein's 'handicap of a head start'. Although Romein's approach is Stalinist in origin, elements of it are compatible with Trotsky's interpretation. But, even an expanded version of the 'law' of uneven and combined development lacks predictive value, although one can say with certainty in hindsight whether a combined development has taken place. It is argued that the 'law' is, in fact, an underspecified social mechanism and that its explanatory power can be increased by identifying a number of recurrent patterns.