Obesity contributes to worse outcomes following treatment using assisted reproductive technology (ART), including lower pregnancy and live birth rates and a higher miscarriage rate. This could be mediated through a detrimental effect on oocyte and embryo quality. Oocyte donation studies have suggested that endometrial changes also play a significant role. Endocrine changes associated with obesity, including hyperandrogenism and insulin resistance, as well as changes in local levels of IGFs, cytokines and leptin are thought to underlie the relationship between raised BMI and adverse ART outcomes. This review aims to highlight these underlying mechanisms.