This paper provides a comprehensive overview on the properties of electrospun nanofibers and their application as reinforcements in composites. The paper first introduces the remarkable properties of electrospun nanofibers including high aspect ratio and molecular orientation, large specific surface area, small pore size, as well as excellent mechanical performance. Next the fabrication methods for the electrospun nanofiber reinforced composites are described. Then different kinds of electrospun nanofiber reinforced composites are discussed in terms of the classifications of electrospun nanofibers. After that, the influences of the mechanical performance of fibers, fiber diameter, fiber amount, fiber/matrix interfacial interaction and the distribution of fibers in the matrix on the reinforcement of composites are discussed. At the end, the possible future challenges and conclusions for electrospun nanofiber reinforced composites are highlighted.