Current practice dictates the separation of the hardware and software development paths early in the design cycle. These paths remain independent with very little interaction occurring between them until system integration. In particular, hardware is often specified without fully appreciating the computational requirements of the software. Also, software development does not influence hardware development and does not track changes made during the hardware design phase. Thus, the ability to explore hardware/software tradeoffs is restricted, such as the movement of functionality from the software domain to the hardware domain (and vice-versa) or the modification of the hardware/software interface. As a result, problems that are encountered during system integration may require modification of the software and/or hardware, resulting in potentially significant cost increases and schedule overruns. To address the problems described above, a co-operative design approach - one that utilizes a unified view of hardware and software - is described. This approach is called hardware/software codesign. This book develops several fundamental hardware/software codesign concepts and a methodology that supports them. A unified representation, referred to as a decomposition graph, is presented which can be used to describe hardware or software using either functional abstractions or data abstractions. Using a unified representation based on functional actions, an abstract hardware/software model has been implemented in a common simulation environment called ADEPT (Advanced Design Environment Prototyping Tool). This model permits early hardware/software evaluation and tradeoff exploration. Techniques have been developed which support the identification of software bottlenecks and the evaluation of design alternatives with respect to multiple metrics. The application of the model is demonstrated on several examples. A unified representation based on data abstractions is also explored.