The authors discuss gigabit transmitter/receiver GaAs ICs for optical communications, focusing on their circuit design architecture and performance. The key technologies for the gigabit transmitter/receiver ICs are discussed and a design architecture for them based on these key technologies is proposed. This architecture is applied to fabricate transmitter/receiver ICs (high-sensitivity preamplifier IC and high-power laser diode driver IC) using a high-transconductance MESFET process. These GaAs ICs have been successfully applied to a 2.4-Gb/s transmitter receiver, which has demonstrated good performance with a minimum average received optical power -31.8 dBm.<<ETX>>