The population for this study is adult men 18-65 years of age inclusive, with primary or secondary hypogonadism, with historical documented total serum testosterone concentration of ≤300 ng/dL and the ability to provide informed consent. Eligible subjects include treatment-naïve, hypogonadal patients with a documented confirmation of hypogonadism, as well as patients previously treated with an alternate topical testosterone replacement therapy (TRT) for at least three months prior to selection. Participants currently receiving topical testosterone replacement therapy will be required to discontinue their current testosterone treatment before initiating treatment with NATESTO. The approximate total duration of study participation for participants completing the study will be up to 150 days (~21 weeks).