Compound semiconductor optoelectronic devices are of great importance for a wide range of applications, such as remote sensing, imaging technology, optical signal processing, optical recording, optical interconnects, and fiber optical communications. There are many optoelectronic devices, but they can be classified into three categories: transmitters, receivers, and switching devices. The optical transmitters include the light-emitting diodes (LEDs), semiconductor laser diodes (LDs), and liquid crystal display devices. The optical receivers include PIN and metal—semiconductor—metal (MSM) photodetectors, avalanche photodiodes (APDs), charge-coupled devices (CCDs), and infrared photodetectors (IPs). The switching devices include nonlinear optical switching devices, acoustic—optic devices, and electrooptic and electroabsorptive guided wave devices. The chapter focuses on compound semiconductor lasers and APDs for optical interconnections; and optical telecommunication systems and quantum well infrared photodetectors (QWIPs) for infrared detector array imaging applications. The chapter discusses the temperature dependences, especially the LT dependences, of compound semiconductor lasers, APDs, and QWIPs. The compound semiconductor lasers and APDs are very promising for modern long-haul, high-speed fiber optical telecommunications. Compound semiconductor lasers are also used widely in applications such as optical recording, optical signal processing, and cryogenic optical data link or optical interconnections. Currently, QWIPs dominate the modern infrared imaging systems operating at low temperatures.