Most electronic systems, whether self contained or embedded, have a predominant digital component consisting of a hardware platform which executes software application programs. Hardware/software co-design means meeting system level objectives by exploiting the synergism of hardware and software through their concurrent design. Co-design problems have different flavors according to the application domain, implementation technology and design methodology. Digital hardware design has increasingly more similarities to software design. Hardware circuits are often described using modeling or programming languages, and they are validated and implemented by executing software programs, which are sometimes conceived for the specific hardware design. Current integrated circuits can incorporate one (or more) processor core(s) and memory array(s) on a single substrate. These "systems on silicon" exhibit a sizable amount of embedded software, which provides flexibility for product evolution and differentiation purposes. Thus the design of these systems requires designers to be knowledgeable in both hardware and software domains to make good design tradeoffs. The paper introduces various aspects of co-design. We highlight the commonalities and point out the differences in various co-design problems in some application areas. Co-design issues and their relationship to classical system implementation tasks are discussed to help develop a perspective on modern digital system design that relies on computer aided design (CAD) tools and methods.