Male aging is characterized by a unique physiological trajectory distinct from female menopause. While menopause signifies the end of reproductive capability in women, aging men typically retain androgen production and fertility until advanced age. Understanding the physiology of male aging and the pathophysiology of Late-Onset Hypogonadism (LOH) is crucial. The hypothalamic-pituitary-gonadal (HPG) axis plays a central role in regulating male gonadal function. During various life stages, from intrauterine development to senescence, the HPG axis undergoes intricate changes, influencing sexual development, reproduction, and overall health. This chapter provides insights into the physiology of male aging, the intricate workings of the HPG axis, the role of testosterone in different life stages, and the pathophysiology of AOH. Understanding these concepts is crucial for clinicians to recognize and address the unique health needs of aging men. Further research is warranted to elucidate the epidemiology, pathophysiology, and optimal management strategies for AOH, aiming to improve the overall health and quality of life in aging males.