Defect concentrations in molecular beam epitaxial (MBE) GaAs range from 1012 to 1020 cm-3 as the growth temperature is lowered from 600 to 200 °C; however, very high quality layers can be grown over this whole range. The dominant defect is the As antisite, but there is also good evidence for As interstitials and gallium vacancies. The particular form of the As antisite center in low—temperature (LT) MBE GaAs is not known at this time, but it is definitely not EL2, because both the thermal activation energy and the electron capture cross section differ significantly. However, other features, such as the EPR spectrum and metastable—to—normal recovery kinetics are identical to those of EL2. The donor (As antisite) to acceptor ratio seems to hold at about one order of magnitude as growth temperature is varied from 200 — 400 °C; thus, the Fermi level stays near mid—gap over this whole range. However, hopping conduction among the As antisite centers is strong for samples grown between 200 and 300 °C and keeps the material from being semi—insulating, while for those grown between 350 — 480 °C, the resistivity is greater than 107 Ω cm. The annealing dynamics are particularly interesting and include such features as the mobility going through a sharp maximum at an annealing temperature of 400 °C for a layer grown at 200 °C. The donor and acceptor concentrations can be determined both by Hall effect and absorption measurements as the layer is annealed up to 600 °C. Above 550 °C, large precipitates are formed. The relative roles of the precipitates and point defects in influencing compensation, lifetime, and device characteristics are a source of much controversy and will be discussed.