Since their inception more than thirty years ago, field-programmable gate arrays (FPGAs) have grown more complex, more capable, and more diverse in their applications. FPGAs can be reprogrammed at a fundamental level, changing the function and interconnection of millions of elements. By reconfiguring their hardware to match the application, FPGAs often achieve higher energy efficiency, lower latency or faster time-to-market across a very wide range of application domains. A modern FPGA combines many components, from logic blocks, programmable routing and memory blocks to networks-on-chip and processor subsystems. For best efficiency, each component must be carefully architected to match the needs of a wide range of applications, and to mesh well with the other components. Their design involves many different choices starting from the high-level architectural parameters down to the transistor-level implementation details. This chapter describes the evolution of these FPGA components, their design principles and implementation challenges.