Endometriosis and PolyCystic Ovary Syndrome (PCOS) are both oestrogen-related diseases that often occur among women of reproductive age. Both have a negative impact on fertility. To date, there is no extensive literature on the association of endometriosis with PCOS. In this chapter, we will try to highlight the possible correlation between the two disorders, as well as to examine possible common pathogenetic mechanisms, as well as to propose a strategy for the joint treatment of the two above-mentioned disorders when they coexist.