Every bit of information that circulates the internet across the globe is a pulse of light, that at some point will need to be converted to an electric signal in order to be processed by the electronic circuitry in our data centers, computers, and cell phones. Photodetectors (PD’s) perform this conversion with ultra high speed and efficiency, in addition to being ubiquitously present in many other devices ranging from the mundane TV remote controls, to ultra high resolution instrumentation used in Laser Interferometer Gravitational Wave Observatory (LIGO) that reach the edge of the universe and measure gravitational waves. The second edition of "Photodetectors" fully updates the popular first edition with updated information covering the state-of-the-art in modern photodetectors. The 2nd edition starts with basic metrology of photodetectors and common figures-of-merit to compare various devices. It follows with chapters that discuss single-photon detection with Avalanche Photodiodes; organic photodetectors that can be inkjet printed; and silicon-germanium PDs popular in burgeoning field of Silicon Photonics. Internationally recognized experts contribute chapters on one-dimensional, nanowire, PDs as well as high speed zero-dimensional, quantum dot, versions that increase the spectral span as well as speed and sensitivity of PDs and can be produced on various substrates. Solar-blind PDs that operate in harsh environments such as deep space, or rocket engines, are reviewed and new devices in GaN technology . Novel Plasmonic PDs, as well as devices which employ micro-plasma of confined charge in order to make devices that overcome speed limitation of transfer of electronic charge, are covered in other chapters. Using different, novel technologies, CMOS compatible devices are described in two chapters, and ultra high speed PDs that use low-temperature-grown GaAs (LT-GaAs) to detect fast THz signals are reviewed in another chapter. Photodetectors used in application areas of Silicon-Photonics and Microwave-Photonics are reviewed in final chapters of this book. All chapters are of a review nature, providing a perspective of the field before concentrating on particular advancements. As such, the book should appeal to a wide audience that ranges from those with general interest in the topic, to practitioners, graduate students and experts who are interested in the state-of-the-art in photodetection.