Androgen resistance causes the androgen insensitivity syndrome in its variant forms and is a paradigm of clinical syndromes associated with hormone resistance. In its complete form, the syndrome causes XY sex reversal and a female phenotype. Partial resistance to androgens is a common cause of ambiguous genitalia of the newborn, but a similar phenotype may result from several other conditions, including defects in testis determination and androgen biosynthesis. The biological actions of androgens are mediated by a single intracellular androgen receptor encoded by a gene on the long arm of the X chromosome. Mutations in this gene result in varying degrees of androgen receptor dysfunction and phenotypes that often show poor concordance with the genotype. Functional characterization and three-dimensional modelling of novel mutant receptors has been informative in understanding the mechanism of androgen action. Management issues in syndromes of androgen insensitivity include decisions on sex assignment, timing of gonadectomy in relation to tumour risk, and genetic and psychological counselling.