We are proposing a formalism to express performance constraints at a high level of abstraction. The formalism allows specifying design performance constraints even before all low level details necessary to evaluate them are known. It is based on a solid mathematical foundation, to remove any ambiguity in its interpretation, and yet it allows quite simple and natural specification of many typical constraints. Once the design details are known, the satisfaction of constraints can be checked either by simulation, or by formal techniques like theorem proving, and, in some cases, by automatic model checking.