Molecular beam epitaxy (MBE) GaAs layers grown at substrate temperatures of 200-400-degrees-C, well under the normal growth temperature of about 600-degrees-C, exhibit unique electrical and optical properties and can be profitably used as buffer layers for metal-semiconductor field effect transistors (FETs), gate-insulator layers for metal-insulator-semiconductor FETs, and channel layers in fast photoconductive switches. Other III-V materials, such as InP, GaP, AlGaAs, InAlAs, InGaAs, and InGaP, have also been grown at low temperatures (LTs) and are, in some cases, useful in device applications. The most outstanding features of LT MBE GaAs are very high point-defect densities, e.g. about 10(20) As(Ga) centers cm-3 in 200-degrees-C material, and high concentrations (about 10(17) cm-3) of large (about 30 angstrom diameter) As precipitates after annealing such material for 10 min at 600-degrees-C. However, there is still much speculation and controversy regarding the relative roles of the point defects and precipitates in determining the optical and electrical characteristics of LT GaAs. Topics discussed here include growth, structural properties, point defects, annealing properties, As precipitates, and devices.