Book excerpt: This book represents an attempt to treat three aspects of digital systems, design, prototyping and customization, in an integrated manner using two major technologies: VHSIC Hardware Description Language (VHDL) as a modeling and specification tool, and Field-Programmable Logic Devices (FPLDs) as an implementation technology. They together make a very powerful combination for complex digital systems rapid design and prototyping as the important steps towards manufacturing, or, in the case of feasible quantities, they also provide fast system manufacturing. Combining these two technologies makes possible implementation of very complex digital systems at the desk. VHDL has become a standard tool to capture features of digital systems in a form of behavioral, dataflow or structural models providing a high degree of flexibility. When augmented by a good simulator, VHDL enables extensive verification of features of the system under design, reducing uncertainties at the latter phases of design process. As such, it becomes an unavoidable modeling tool to model digital systems at various levels of abstraction.