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Consider a sequence of markets for goods and securities at successive dates, with no market at any date complete in the Arrow-Debreu sense. A concept of common expectations is proposed that requires traders to associate the same future prices to the same future exogenous events, but does not require them to agree on the (subjective) probabilities associated with those events. An equilibrium is a set ofprices at the first date,a set ofcomrnon price expectations for the future, and a consistent set of individual plans for consumers and producers such that, given the current prices and price expectations, each individual agent's plan is optimal for him, subject to an appropriate sequence of budget constraints. The existence of such an equilibrium is demonstrated under assumptions about technology and consumer preferences similar to those used in the typical Arrow-Debreu theory of complete markets. However, an equilibrium can fail to exist if some provision is not made for the elimination of "unprofitable" enterprises. The usual assumptions of "rationality" imply, in this model, that agents learn from experience and modify their expectations as Bayesians. 'The research on which this paper is based was supported in part by the National Science Foundation. A preliminary version of this was prepared while I was an Overseas Fellow at Churchill College, Cambridge, 1969-70, and I would like to thank the Master and Fellows of Churchill College and the Faculty of Economics of Cambridge for their facilitation of my research during that period. 289