Endometriosis is a benign, estrogen-dependent disease with an obscure etiology. Even the most widely accepted theory of retrograde menstruation cannot satisfactorily explain the development of endometriosis due to the many gaps in our understanding of its pathophysiology. Although most women have retrograde menstruation; only some develop endometriosis. Apart from simply being present in the peritoneal cavity, the endometrial cells are able to attach to, invade the peritoneum, and proliferate to create and maintain an endometriotic lesion. Matrix metalloproteinases (MMPs) are a group of enzymes that degrade the extracellular matrix. These enzymes participate in the histologic changes of the endometrium during the menstrual cycle with generally a higher expression during the menstrual and proliferative phase of the endometrium and a decreased expression during the secretory phase. As noted above, not only do these enzymes play a crucial factor in the cycling endometrium but the degradation of extracellular matrix is essential for the endometrial cells to invade the peritoneum and to develop an endometriotic lesion as well. The aim of this review is to describe the altered expression of MMPs in the development of endometriosis.