Superconducting nanowire single photon detectors (SNSPDs) are excellent detectors for light from the UV to the mid-IR. These cryogenically cooled detectors can measure individual particles of light with high system detection efficiency (the record is over 99%), picosecond-level timing resolution and sub-hertz dark counts. I will describe recent technological developments in SNSPDs including operation in the mid-IR range, kilo-pixel detector arrays, and photon number resolution. These advances mean that SNSPDs are being used in an increasing number of applications including deep space optical communication, quantum information, dark matter searches, biomedical imaging, and astrobiology.