SNSPD is significantly faster than conventional transition edge sensors and operates at higher temperatures. A degree of photon-number resolution can be achieved in SNSPD arrays, through time-binning or advanced readout schemes. Most SNSPDs are made of sputtered niobium nitride (NbN), which offers a relatively high superconducting critical temperature (≈ 10 K) which enables SNSPD operation in the temperature range 1 K to 4 K (compatible with liquid helium or modern closed-cycle cryocoolers). The intrinsic thermal time constants of NbN are short, giving very fast cooling time after photon absorption (<100 picoseconds). The absorption in the superconducting nanowire can be boosted by a variety of strategies: integration with an optical cavity, integration with a photonic waveguide or addition of nanoantenna structures. SNSPD cavity devices in NbN, NbTiN, WSi & MoSi have demonstrated fibre-coupled device detection efficiencies greater than 98% at 1550 nm wavelength with count rates in the tens of MHz. The detection efficiencies are optimized for a specific wavelength range in each detector. They vary widely, however, due to highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. SNSPD devices have also demonstrated exceptionally low jitter – the uncertainty in the photon arrival time – as low as 3 picoseconds at visible wavelengths. Timing jitter increases as photon energy drops and has been verified out to 3.5 micrometres wavelength. Timing jitter is an extremely important property for time-correlated single-photon counting (TCSPC) applications. Furthermore, SNSPDs have extremely low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon. In addition, the deadtime (time interval following a detection event during which the detector is not sensitive) is on the order of a few nanoseconds, this short deadtime translates into very high saturation count rates and enables antibunching measurements with a single detector. For the detection of longer wavelength photons, however, the detection efficiency of standard SNSPDs decreases significantly. Recent efforts to improve the detection efficiency at near-infrared and mid-infrared wavelengths include studies of narrower (20 nm and 30 nm wide) NbN nanowires as well as extensive studies of alternative superconducting materials with lower superconducting critical temperatures than NbN (tungsten silicide, niobium silicide, molybdenum silicide and tantalum nitride). Single photon sensitivity up to 10 micrometer wavelength has recently been demonstrated in a tungsten silicide SNSPD. Alternative thin film deposition techniques such as atomic layer deposition are of interest for