The Digital Library of Mathematical Functions (DLMF) is an online reference on the properties of the special functions of applied mathematics. It contains three methodological chapters and 33 chapters focused on individual classes of functions. Information on functions include definitions, notations, series expansions, integrals, connection formulae, asymptotics, special values, methods of computation, etc. It includes hundreds of visualizations of the functions, including interactive three-dimensional representations of complex-valued functions. A math-aware search engine is also available. Extensive links to available software and external references are also provided.