The effects of external and internal strains and of defect charges on the formation of vacancies, antisites and interstitials in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that strain and doping permit a defect engineering of III-V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while vacancies may be favored only by extreme conditions of compressive strain. Interstitials may be moderately favored by doping and tensile strain. (C) 2001 Elsevier Science B.V. All rights reserved.