Two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. The low temperature operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs) was studied, demonstrating that they can operate with an extremely low dark count rate, which opens up a host of new applications. Secondly, superconducting nanowire single-photon detectors (SNSPDs) based on amorphous materials were developed, fabricated and characterized. These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. Finally, it was demonstrated that the use of NFADs can enable record distance quantum key distribution (QKD) over 307 km of optical fiber. A next generation QKD system has been proposed and numerically analyzed, aiming at extending the record distance by an additional 100 km and increasing the state-of-the-art secret key rate by over an order of magnitude.