We begin our discussion with an overview of Phototransduction phototransduction utilizing Semiconductor semiconductor devices. This is a vast topic that spans several fields of study. Therefore, in this chapter, we provide only the fundamental notions that are necessary to understand the operation of a Photodetector photodetector and the rationale for the techniques adopted in its design. We first discuss the physical mechanisms underlying the conversion of photons to Free carriers free carriers in a Semiconductor semiconductor material. This process, which is referred to as Phototransduction phototransduction, is at the core of Imaging imaging science, and a thorough understanding of its nature is required when designing Front-end front-end and readout chain circuits. We center our discussion on a set of Optoelectronic optoelectronic performance metrics which, when taken together, are an efficiency measure of Phototransduction phototransduction for a given Photosensor photosensor system. Building upon the theory of Phototransduction phototransduction, we introduce the state-of-the-art Front-end front-end and Readout circuit readout circuits associated with Single-photon avalanche diode (SPAD) single-photon avalanche diodes (SPADs). This chapter is largely based on its related publication (Dandin, CMOS Single-Photon Avalanche Diodes and Micromachined Optical Filters for Integrated Fluorescence Sensing, Ph.D. dissertation, University of Maryland, College Park, College Park, Maryland, 2012).