Endometriosis is a common gynecological disorder with varied symptomatology including chronic pelvic pain, dysmenorrhea, and infertility. The association of endometriosis and infertility has been recognized for years, although definite evidence of causality still eludes us. In this review, we will explore three general concepts that enhance our understanding of the cellular and molecular interactions contributing to the pathophysiology of this disorder and that have steered current research in endometriosis. First, we review evidence of a local peritoneal inflammatory process, supported by the findings of elevated cytokine and growth factor concentrations in peritoneal fluid of affected patients. Second, we propose a role for angiogenic factors in the establishment of ectopic implants. Third, we review evidence for biochemical differences of eutopic and ectopic endometrium in endometriosis patients, which may contribute to both the pathogenesis and sequelae of this important disorder. Through information derived from these research efforts, we hope to develop better therapeutic interventions as adjunctive or alternative therapies to our current medical and surgical armamentarium.