A significant thrust in optical metrology is the study of processes that liberate extremely low-amplitude optical signals. Traditionally, photomultiplier tubes have been used, although, as this tutorial will discuss, many applications are being increasingly instrumented with solid-state devices utilizing the avalanche gain multiplication process. Such methods are not new (indeed the history is particularly illuminating), however, the wealth of applications presents the questions of a) where did such devices come from, b) what are the overall performance metrics and bottlenecks for metrology, c) what future innovations can we expect and d) how can we access such technologies with the customisation that may be required for our particular application. We will discuss many modern applications for such single-photon detectors including: photon counting and timing in physical experimentation (with very low noise requirements), the measurement of biological samples using fluorescence lifetimes, their use to decrease costs within positron-emission tomography (PET) systems, the robust measurement of photon arrivals for 3D Imaging and LIDAR, and of course advanced driver-assistance systems (ADAS). We will also consider the use of avalanche gain for optical communications where accurate metrology directly impacts the bit error rate performance. This tutorial aims to give delegates a representative overview of a growing and increasingly commercializedfield and will discuss the future and use of these technologies within the metrology community.