This chapter begins with a recapitulation of an optical link and what the general requirements of an optical receiver are. The discussion of optical receivers starts with a brief analysis of a passive current-voltage converter (i.e., a resistor) in terms of its gain, bandwidth and input-referred noise. This section proposes reasonable bandwidth requirements for optical receivers that do not use equalizers, so-called low-ISI systems. Open-loop and feedback amplifiers are considered. Additional amplification through main-amplifier design is explained, starting with the effect on bandwidth of cascading multiple first-order stages. Behaviour of second- and third-order systems are also presented. Examples of Cherry-Hooper, second-order active feedback and third-order active feedback as well as interleaving feedback are presented. CMOS inverter-based designs are discussed.