Hypogonadism is present in 33% male patients with obesity and/or type 2 diabetes. These patients have low total and free testosterone concentrations, along with inappropriately normal or low LH and FSH concentrations. Hence, they have hypogonadotropic hypogonadism (HH). Testosterone concentrations have an inverse relationship with body weight but are not related to HbA1c or the duration of diabetes. The prevalence of HH increase with increasing BMI in non-diabetic males as well. This inverse relationship is maintained regardless of age. Around 75% of severely obese young males between 14 and 20 years of age exhibit HH. Following bariatric surgery, testosterone levels normalized after 2 years. Thus, body weight is a major determinant of plasma testosterone concentrations. Diabetes patients with HH also have increased insulin resistance compared to those without. Testosterone therapy restores insulin sensitivity, while simultaneously reducing adiposity and increasing muscle mass. Consistent with these observations, testosterone prevents the development of diabetes. This chapter reviews the information on this association, the clinical consequences of HH in these men and the results of replacement with testosterone.