Late-onset hypogonadism (LOH) is defined as the age-related failure of the hypothalamic-pituitary-testicular (HPT) axis which eventually leads to testosterone (T) deficiency. Although aging has a direct detrimental role on the HPT axis, it is also a marker of the accumulation of damages due to age-related conditions, including obesity, diabetes mellitus, and other chronic illnesses. LOH should be suspected in subjects with features of androgen deficiency; however, these may not be easily recognizable, and they are often nonspecific with wide overlap with features of aging and comorbidities. According to the current literature, decreased sexual desire, erectile dysfunction, and impaired morning erections are the most specific symptoms of LOH. In men suspected of LOH, total T should be measured with a reliable method and with appropriate pre-test conditions. The measurement of sex hormone–binding globulin (SHBG) to calculate free T may add information, although the actual usefulness of this is still debated. Although not included in the diagnostic criteria, other biochemical markers may help in increasing the specificity of the diagnosis of LOH. LOH is a functional condition; however, in selected cases with particularly low total T or particularly low or high gonadotropins, an organic origin should be excluded using pituitary magnetic resonance imaging, testicular ultrasound, or karyotype.