Until the 2000s Testosterone (T) Replacement Therapy (TRT) wasn't very satisfactory for male hypogonadic patients because the available T formulations weren't able to reproduce the physiological pattern of T secretion in man. In fact, oral formulations (oral undecanoate T) showed very short half-life (<24 hours), requiring the administration of several daily doses, whereas the old injection products (T esters) were characterized by very long half-life (>7 days) because of their adipose tissue storage, requiring to be administered every 2-3 weeks but determining remarkable and quick fluctuations (in 2-3 weeks) of the testosteronemia with variations in a few days from over-physiological levels (> 2000 ng/dl) to very low levels (< 200 ng/dl). Nowadays, several compounds can attain the standards of suitability and effectiveness of TRT in hypogonadal men. Both transcutaneous (gel) T and long-acting injectable formulations are the most modern preparations that can satisfy the criteria of an ideal chronic replacement therapy. In fact, they keep the serum T levels in the physiological range imitating its circadian rhythm, leading to the development and/or the preservation of male sexual characteristics and, finally, positively influencing bone mass, skeletal muscle and adipose tissue distribution. In particular, the availability and use of long-acting injectable undecanoate T can really improve the patients' compliance as requested for a life-long treatment. However, definitive and conclusive evidence regarding the main end-points, such as the diminished recurrence of falls in elderly men, the decrease in fractures in osteoporotic subjects, the reduction in disabling conditions and the extension of life, have not been reached so far. Therefore, the aim of this review is to sum up the most important evidence that has been collected regarding TRT, highlighting in particular those concerning both transcutaneous and long-acting injectable T compounds.