Testosterone plays a critical role in sexual function, muscle growth and bone mineralisation, and has important behavioural effects in men. Levels of circulating testosterone decline by approximately 1% annually from the age of 40 years onwards. However some of the symptoms of low testosterone are non-specific, and may be caused by co-morbidities rather than low testosterone itself. Safety concerns have been raised regarding risks of cardiovascular events and prostatic growth in men during testosterone therapy. There is also an increasing public awareness of age-related low testosterone in men, which is causing an increase in prescribing of testosterone replacement in primary care. Testosterone replacement is clearly beneficial when levels of testosterone are very low, and patients are young enough to have minimal exposure to any adverse effects. However, in older men, or men with co-morbidities or borderline levels of testosterone, the endocrinologists face difficult decisions and confusion about current evidence. This session use real cases to give practical tips on assessing patients with low testosterone and selecting patients for testosterone replacement. It will also provide a concise update on the evidence and clinical guidelines in this core but controversial aspect of endocrine practice.