Self-contained and pedagogical introduction to key string theory ideas and techniques needed to understand D-branes. In string theory, D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named. D-branes are typically classified by their spatial dimension, which is indicated by a number written after the "D." A "D0-brane" is a single point, a "D1-brane" is a line (sometimes called a "D-string"), a "D2-brane" is a plane, and a "D25-brane" fills the highest-dimensional space considered in "Bosonic string theory." There are also "instantonic D(–1)-branes," which are localized in both space and time. D-branes were discovered by Dai, Leigh and Polchinski, and independently by Hořava in 1989. Author Biography Clifford V. Johnson obtained his BSc in physics at Imperial College, University of London, and his PhD in theoretical physics from the University of Southampton. He is currently Senior Lecturer in Theoretical Physics, Department of Mathematical Sciences, University of Durham, where he is part of the Centre for Particle Theory.