The action systems framework has recently been applied to the area of synchronous VLSI design. In this paper, we present a set of concepts necessary in the formal design of synchronous hardware, using synchronous action systems. These explicitly model synchronous two-phase behavior. They are developed using the refinement calculus in a usual manner. The fundamental idea of synchronous action systems is to make a formal description transparent in the sense that its hardware implementation can be obtained by direct mapping. The presented design methodology also allows a postponed decision on the final implementation aspects regarding the architecture (synchronous / asynchronous)