This chapter discusses the basics of Complex programmable logic devices (CPLDs) and field programmable gate arrays (FPGAs) and presents an introduction to design methodology for digital systems. Programmable logic devices (PLDs) are represented in three general forms: programmable read-only memories (PROMs), programmable logic arrays (PLAs), and programmable array logics (PALs). One of the most popular hardware description languages used for application-specific integrated circuits (ASICs) and FPGA/CPLD-based digital system design is Very high speed integrated circuit (VHSIC) hardware description language (VHDL). The chapter also presents an example of a digital system to demonstrate the design flow for FPGA-based system design. A pulse-width measurement system is to be designed to measure the pulse width in the range of 1 ms to 300 ms with a resolution of 5 ns. FPGAs are a popular target platform in automotive and consumer electronic products as well as medical applications, as low-cost development boards are now available from FPGA manufacturers.