This chapter introduces a family of silicon devices, FPGAs, exploring their features and applications, while giving some basic hints about their architecture. We will discuss the low-level configuration details of these devices, so part of this chapter will be devoted to explore the file type that contains the configuration information to initialize the state of these reconfigurable chips, the bitstream file. For describing the bitstream composition, the details have been aggregated from the three different families of FPGAs examined in this book, namely the Spartan-3, Virtex-II Pro, and Virtex 4 families manufactured by Xilinx Inc.