Throughout the 1950s and 1960s, Jacques Tits develops one of the most original constructions in 20th-century mathematics: the theory of buildings, abstract geometric complexes specifically designed to give a visual, intuitive interpretation to a very broad class of algebraic groups — including simple algebraic groups and finite groups of Lie type — that could otherwise only be described through dense abstract algebra. Drawing inspiration from Felix Klein's Erlangen program, which linked geometry and group theory, Tits shows that these groups can be understood much more richly as the automorphism group — the symmetry group — of a carefully constructed building, giving rise to an additional algebraic structure known as a BN-pair or Tits system, and to the corresponding Bruhat decomposition of the group. These buildings turn out to be, in essence, extremely high-dimensional generalizations of familiar geometric objects such as regular polytopes, and allow entire families of groups sharing the same underlying combinatorial structure to be classified and visually understood. Tits's theorem on group growth — establishing when a finitely generated subgroup of a Lie group is virtually nilpotent — would become decades later an essential technical ingredient of Mikhail Gromov's work on groups of polynomial growth, directly intertwining the contributions of both mathematicians, who would also coincide as co-laureates of the 1993 Wolf Prize in Mathematics. Tits's work reaches far beyond pure algebra: his geometric language has become a standard tool in algebraic group theory, differential geometry, and, more recently, in the study of infinite-dimensional Kac-Moody groups.