Optical fibers are at the forefront of several advanced technologies, such as fast communication networks, powerful lasers and accurate sensors. As scientists and engineers push the limits of these technologies, accurate and innovative methods for fiber characterisation are required. Single-photon detectors offer a unique advantage as they combine ultimate sensitivity with excellent timing accuracy (<100 ps), resulting in high tolerance to loss and high spatial resolution (1 cm). In this application note, we will give an overview of how these technologies may be used to build a custom optical time-domain reflectometers from benchtop instruments. Adapting the OTDR system to the user’s desired measurement, as well as an appropriate choice of detector, will result in significantly higher performance than what can be obtained with commercial OTDRs.