A clear picture of the dynamic relationship between the endometrium and peritoneum is emerging as both tissues may participate in the spontaneous development of endometriosis. Various adhesion molecules, pro-inflammatory cytokines and chemoattractants cytokines have emerged as central coordinators of endometrial-peritoneal interactions. The peritoneal microenvironment which consists of the peritoneal fluid, normal peritoneum and peritoneal endometriotic lesions may play an active role in the pathogenesis of endometriosis, by harbouring most inflammatory responses that are triggered by the presence of endometrial cells, leading to recruitment of activated macrophages and leukocytes locally. Menstrual endometrium has the ability to bond and invade the peritoneal tissue. In baboons intrapelvic injection of menstrual endometrium permits the study of early endometrial-peritoneal interaction in an in vivo culture microenvironment and can lead to important insight in the early development of endometriotic lesions. In this review, we discuss the roles of the endometrial-peritoneal interactions, not only in disease development but also in the broader process of aetiopathogenesis.