Abnormal human spermatogenesis can be caused by defects in androgen action because of androgen insensitivity. A variety of mutations have been described in the human androgen receptor gene associated with male infertility. These can be attributed to two molecular mechanisms. First, point mutations in the androgen receptor gene cause alterations in the amino acid sequence and, hence, lead to apparently slight changes in the androgen receptor effector mechanisms and mild androgen insensitivity. Secondly, variations in the polymorphic poly glutamine segment within the N-terminal end of the androgen receptor have been ascribed to correlate with fertility aspects possibly because of modifications of transcriptional regulatory mechanisms. It has been postulated that longer poly glutamine segments are associated with decreased sperm counts. However, the molecular mechanisms that lead to inhibition of spermatogenesis because of a mutated androgen receptor are poorly understood and will need more focus in the future.