Surface plasmon photodetectors are of vigorous current interest. Such detectors typically combine a metallic structure that supports surface plasmons with a photodetection structure based on internal photoemission or electron-hole pair creation. Detector architectures are highly varied, involving surface plasmons on planar metal waveguides, on metal gratings, on nano-particles, -islands, or -antennas, or involving plasmon-mediated transmission through one or many sub-wavelength holes in a metal film. Properties inherent to surface plasmons, such as sub-wavelength confinement and their ability to resonate on tiny metallic structures, are exploited to convey useful characteristics to detectors in addressing applications such as low-noise high-speed detection, single-plasmon detection, near- and mid-infrared imaging, photovoltaic solar energy conversion, and (bio)chemical sensing. The operating principles behind surface plasmon detectors are reviewed, the literature on the topic is surveyed, and avenues that appear promising are highlighted.