Field programmable gate arrays (FPGA) are a recently developed family of programmable circuits. Like mask programmable gate arrays (MPGA), FPGAs implement thousands of logic gates. But, unlike MPGAs, a user can program an FPGA design as traditional programmable logic devices (PLDs): in-site and a in a few seconds. These features, added to reprogrammability, have made FPGAs the dream tool for evolvable hardware. This paper is an introduction to FPGAs, presenting differencies with more traditional PLDs and giving a survey of two commercial architectures.