A flux of weakly interacting neutral particles incident on the Earth would produce occasional nuclear-recoil events in any material. These could be detected as scintillation or ionization pulses in suitable target materials. Expected rates are in the range 1–10–4events kg–1d–1. These must be distinguished from much higher rates due to backgrounds from gamma and beta background, even in deep underground locations. Methods of uniquely identifying nuclear recoils are described using crystal scintillators, scintillation and ionization processes in liquid xenon, and ionization tracks in gases. World progress on these techniques and future prospects are summarized. A possible future weakly interacting massive particle detector based on mechanical recoil of suspended microgranules is proposed which, with advances in nanotechnology, could eventually be extended to the detection of low-energy relic neutrinos.