This book covers the latest trends in the design of single-photon avalanche diodes (SPADs), which are the front-end sensors in modern photon counting systems. The authors describe the fundamental physics that enable photon counting in these devices. They also discuss systems that are made from these detectors, specifically describing circuit architectures that may be used to achieve high-fidelity photon counting. Coverage features example devices and systems designed in the authors’ research groups as well as different approaches undertaken by other experts in the field. The authors take a unique, modular approach that covers every aspect of the design stack, with stand-alone chapters, allowing readers to focus on specific aspects of the technology stack. Coverage includes the device-physics aspects of the detectors, their integration in modern electronics fabrication technologies like CMOS, and application-specific systems that utilize these detectors.