Adiposity is associated with reduced fertility in men. The aetiology is multifactorial, with obese men at greater risk of suffering from impaired spermatogenesis, reduced circulating testosterone levels, erectile dysfunction and poor libido. The diagnosis and treatment of reduced fertility observed in obese men therefore requires insight into the underlying pathology, which has hormonal, mechanical and psychosocial aspects. This article summarises the current epidemiological, experimental and clinical trial evidence from the perspective of a practicing clinician. The following conclusions and recommendations can be drawn: Obesity is associated with low serum testosterone concentrations, but treatment with exogenous testosterone is likely to adversely impact on fertility. It is important to discuss this with men prior to initiation of testosterone therapy. Obesity adversely affects sperm concentration and may affect sperm quality. However, whether or not weight loss will correct these factors remain to be established. Oestrogen receptor modulators (and aromatase inhibitors) are unlicensed in the treatment for male hypogonadism and/or infertility. These treatments should hence be considered experimental approach until ongoing clinical trials report their outcomes.