An outline of infrared localized vibrational mode absorption spectroscopy relevant to silicon impurities in GaAs is presented. Absorption lines from Si,. donors, Si,, acceptors, SiGa-SiAS pairs, SiGa-VGa (Ga vacancy) pairs and a complex Si-X (involving SiAS and VGe) have been identified as well as lines from Si,,-Cu,, Sia,-H, Si,,-H and SiGa-BAs pairs. These observations are related to the electrical properties of n-type Bridgman, liquid-encapsulated Czochralski and molecular beam epitaxial (MBE) (001) GaAs, and to p-type liquid phase epitaxial and MBE (111)A layers. The discussion relates to dynamic site switching, effects due to counter-doping with shallow acceptors, the solubility of silicon, and DX behaviour observed in homogeneously doped material and proposed for S-doped MBE (001) layers. The major problem is to understand the processes that limit the maximum carrier concentration that can be achieved in n-type crystals.