The many roles testosterone plays throughout life are set out in this chapter. It covers the hormone's part in developing characteristics such as hair and muscle growth and a deeper voice, and its influence on bone density, stamina and energy. Readers learn how testosterone drives changes during puberty, why balanced levels matter for sperm production and male fertility, and how in females it serves as a precursor to oestradiol and supports ovarian function and sex drive. The chapter also discusses its effects on mood and cognition in all genders, and explains the gradual decline with age, typically around 1% a year from 30.