During the 1980s British Rail has been addressing itself to the problem of replacing its diesel multiple unit fleet. Initially two approaches of lightweight railbus and full weight diesel-electric multiple unit were tried and subsequently medium weight diesel-mechanical prototypes were evaluated. The paper sets out the principal factors affecting the specification of each of the prototype units, the service experience gained with each type and the system studies and cost evaluations which were carried out. All of this has taken place against a background of a constantly changing business requirement for the size of fleet, the duty and the sector of the market for which the units are intended. The paper shows how these changed requirements have influenced, and are continuing to influence, the specification of the production fleet which is currently based on a mixture of medium and lightweight units.