D-branes (short for Dirichlet branes) are extended objects in string theory on which open strings can end, with their endpoints constrained to move along the brane’s worldvolume. Originally emerging as a mathematical boundary condition in perturbative string theory, D-branes have evolved into central physical entities, carrying dynamical degrees of freedom and coupling naturally to Ramond–Ramond fields. They play a pivotal role in nonperturbative formulations of string theory, providing a unifying framework for gauge theories, black hole microstate counting, and dualities such as the AdS/CFT correspondence. Beyond physics, D-branes have had a profound influence on mathematics, inspiring deep connections with algebraic geometry, category theory, and homological mirror symmetry, and serving as a bridge between physical intuition and abstract mathematical structures.