Photodetectors in optical systems work in a similar manner like human eyes. Optical detectors can detect signals from light sources and provide feedback to the networks. Modern nanoscale semiconductor photodetectors are indispensable components for various high-speed optical networks in the applications of datacenter, wireless, fiber-to-the-premises, and telecommunication. In this chapter, we focus on the state-of-the-art 2.5G, 10G, and 25G avalanche photodiodes and compare the feature size in each generation. We present brief overview of the key device performance of avalanche photodiodes including avalanche breakdown voltage, dark current, temperature stability, bandwidth, and sensitivity. We also discuss reliability implications associated with device miniaturization. During device shrinking, increasingly high electric field is likely to impose most reliability risk. We discuss the reliability challenges of nanoscale photodetectors in terms of optical/electrical overload stress, wear-out degradation, and electrostatic discharge.