This tutorial will present a comprehensive analysis of single-photon detector sensors and systems. The material will range from semiconductor structure, front-end and readout circuit architectures, to intellectual property. Specifically, we will cover device structures that are configured to yield high-performance single photon detection in CMOS platforms, and we will cover readout circuit architectures that allow complex photon counting applications. We will discuss several high-impact applications including silicon photomultiplier configurations for nuclear imaging/detection and integrated single photon arrays for computer vision. The tutorial has three main educational objectives. The first is to cover the fundamental aspects of single photon detector technology in circuits and systems. The second is to identify the latest research trends in the field. And the third is to situate the technology in the commercial landscape.