Planning, uncertainty and economic policy. a formalized procedure: «optimix» The article deals with the problems raised by the coordination of the annual Budgets with medium-term Plan for the case of the French Economy. These problems were made more acute by the increased importance of uncertain uncontrolled factors during the period of the sixth Plan. A formalized procedure for adapting the implementation of the Plan to the occurence of unforeseen events is proposed. It is based on a structuration of the problem in the framework of a classical decision-making scheme. In second part, the elements of this scheme are presented: variables, dynamic model, objective function, uncontrolled factorsf choice criterion, information structure. The numerical results will be presented in a forthcoming article in this review.