  Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs)
have rapidly emerged as a highly promising photon-counting technology for
infrared wavelengths. These devices offer high efficiency, low dark counts and
excellent timing resolution. In this review, we consider the basic SNSPD
operating principle and models of device behaviour. We give an overview of the
evolution of SNSPD device design and the improvements in performance which have
been achieved. We also evaluate device limitations and noise mechanisms. We
survey practical refrigeration technologies and optical coupling schemes for
SNSPDs. Finally we summarize promising application areas, ranging from quantum
cryptography to remote sensing. Our goal is to capture a detailed snapshot of
an emerging superconducting detector technology on the threshold of maturity.
