Hardware implementation aspects of the MD5 hash algorithm are discussed in this paper. A general architecture for MD5 is proposed and several implementations are presented. An extensive study of effects of pipelining on delay, area requirements and throughput is performed, and finally certain architectures are recommended and compared to other published MD5 designs. The designs were implemented on a Xilinx Virtex-II XC2V4000-6 FPGA and a throughput of 586 Mbps was achieved with logic requirements of only 647 slices and 2 BlockRAMs. Methods to increase the throughput to gigabit-level were also studied and an implementation of parallel MD5 blocks achieving a throughput of over 5.8 Gbps was introduced. At least to the authors' knowledge, MD5 designs presented in this paper are the fastest published FPGA-based architectures at the time of writing.