Endometriosis, defined as the presence of functional endometrium, composed of glands and stroma, outside the uterine cavity, is a common benign disease, causing abdominal pain, dysmenorrhea and dyspareunia in about 10–15% of all women in reproductive age and more than 30% of women attended in infertility clinics. There are a number of theories to explain how endometriosis occurs. Out of them, the implantation theory of Sampson (1927) is the most widely accepted. According to Sampson,, the endometrial tissue is retrogradely shed through the Fallopian tubes into the peritoneal cavity, where it attaches and proliferates at ectopic sites during menstruation. However, endometriosis is a condition showing congenital tendencies and a polygenic/multifactorial etiology has been suggested. The cause for the adherence of the eutopic endometrium in ectopic locations remains unknown in these patients. In addition there are several factors like etiopathogenic, genetic, environmental, hormonal, immunologic, oxidative stress and growth factors which are implicated in the establishment, development and maintenance of endometriosis.