A signal undergoes an E/O transformation at a fiber optic transmitter and a corresponding O/E transformation at the receiver. This chapter discusses the O/E process, with emphasis on the first two blocks in that flow: the semiconductor optical detector, or photodiode, and preamplifier. The two together are referred to as the receiver's front-end. With the growing need for commercial systems operating at 10 and even 40 Gb/s, the design of front-ends has become more important and technically challenging.