Male hypogonadism (HG) is defined as impaired testicular function, leading to inadequate testosterone production and/or defective spermatogenesis. Diagnosis relies on serum total testosterone (tT) measurement. The current consensus suggests a threshold of approximately 12 nmol/L (346 ng/dL) as the lower limit of normal, while values <8 nmol/L (231 ng/dL) clearly indicate androgen deficiency requiring treatment. For intermediate values, free testosterone (fT) assessment may provide additional diagnostic accuracy. HG classification is commonly based on the level of dysfunction within the hypothalamic–pituitary–testicular (HPT) axis. Hypothalamic or pituitary disorders cause secondary (hypogonadotropic) HG, whereas testicular failure results in primary (hypergonadotropic) HG; combined forms may also occur. Another framework distinguishes between selective HG, affecting either Leydig cell steroidogenesis or spermatogenesis, and total HG, affecting both. The age of onset further shapes the clinical presentation: fetal onset may lead to differences in sex development, micropenis, or cryptorchidism; childhood onset typically manifests as delayed puberty; adult-onset disease may be subtle, with nonspecific symptoms preceding sexual dysfunction; and late-onset HG (LOH), prevalent in aging men, is characterized by partial testosterone deficiency with gradual progression. Recently, a practical classification of organic and functional HG has been proposed. Organic HG reflects structural pathology within the HPT axis and requires standard treatment, whereas functional HG occurs without demonstrable abnormalities, is often associated with comorbidities, and may be reversible through lifestyle or medical management of underlying conditions. Epidemiological data remain heterogeneous due to variable age ranges and testosterone cutoffs, but the overall prevalence is estimated at 6–12% among men aged 30–70 years, with incidence increasing with age. LOH accounts for most cases, whereas congenital and organic forms are less common. Notably, congenital hypogonadotropic HG occurs in ~1:8000 males, and Klinefelter syndrome, the most common chromosomal abnormality in humans, affects approximately 1 in 650 newborn males.