The diagnosis of testosterone deficit (TD) in adult men is made upon the finding of consistent symptoms or signs associated with confirmed low Testosterone levels. TD can be caused by primary testicular failure, decreased LH secretion (secondary) or a combination of both (mixed type). The clinical features of TD depend on the age of onset: Before completion of pubertal development it results in eunuchoid traits that can be easily detected. The clinical expression of adult onset TD is protean, with varied symptoms and signs often overlapping with those of aging and several chronic conditions. Sexual problems as low desire and difficult erections are frequently reported by TD patients. Several symptom questionnaires have been proposed for detecting TD. However, poor specificity and weak correlation with serum testosterone make them unsuitable as diagnostic tools, although their scores can help for monitoring future treatments. Physical examination usually yields few significant data but it can help for detecting unidentified cases of early onset TD. Measuring serum total testosterone(TT) levels is mandatory It is available in most laboratories although commonly used methods are not fully satisfactory In adult men the lower limit for the normal reference range is around 10 nmol/ L, but this figure can vary among laboratories. "Grey zone" values falling within a +/- 20% interval around this limit need further clarification. To this aim most hospital laboratories can calculate the free (FT) or bioavailable fractions (BioT) of TT, SHBG, and albumin levels. When TD diagnosis is confirmed, - a number of additional tests, such as measuring gonadotropins and prolactin, should be used to find out its etiology They are summarized here in a diagnostic algorithm. There is a need for laboratory tests giving a reliable estimate of the androgen sensitivity of each subject.