Kahn's Fellowship Dissertation, 'The Economics of the Short Period,' is a major contributions to the development of the theory of imperfect competition. The short period has two aspects: the first is the shape of its prime cost curve, the second is the shape of the individual demand curve. By introducing the imperfection of the market, Kahn was able to explain why price does not fall to marginal cost at a low level of demand and why the equilibrium output is less than full capacity. This paper discusses Kahn's contribution and the question of why imperfect competition was not incorporated into 'The General Theory.' (c) 1994 Academic Press, Inc. Copyright 1994 by Oxford University Press.