This paper examines the primary sources of attack against which IP security must be defended in FPGAs, describing the two predominant security methods employed in this defense: one that uses battery-backed volatile memory for encryption key storage and one that uses non-volatile memory. It then provides an explanation of the inherent and standards-fulfilling security features of the Xilinx Virtex-4 and Virtex-5 FPGA families of solutions, which have earned the trust of many government and commercial institutions.