Endometriosis is an enigmatic disease affecting up to 10% of reproductive-aged women causing pain and infertility. Up to 50% of women with endometriosis are infertile, and agreement about treatment options has been difficult to establish. The association between endometriosis and infertility is derived from comparisons of fertile and infertile women, animal models, donor sperm studies, and in vitro fertilization results. Diagnostic approaches based on endometrial changes associated with endometriosis are also providing insights into possible mechanisms of infertility, especially in women with milder forms of the disease. Treatment of endometriosis, including surgical ablation or resection, is cost-effective and offers the potential for improvement in cycle fecundity. Medical management of endometriosis-associated infertility has not been proven outside of in vitro fertilization.