This is a brief scientific biography of Boltzmann, including the vicissitudes of his uncommon life and the principal developments of his works about the kinetic-molecular theory. Boltzmann was a man of broad culture with many interests in literature, poetry, music, mathematics, philosophy, technology, pedagogy, and more; he was a man of action, an efficient administrator, a caring husband and father, a thoughtful colleague, an inborn teacher, and a joyous, humorous companion in informal circumstances. Of these many facets of Boltzmann’s personality, this chapter will examine reflections in the theory that made him most famous.