A realistic superstring theory will have to contain gauge fields for the electroweak and strong interactions and perhaps for a grand unified theory. The freedom to choose the right and left movers of different kinds of string derives from the facts that the states of a bosonic string or of type-II superstring are direct products of the Fock space states for the right and left movers, and that interactions respect this structure. Such a string theory is referred to as a heterotic string. This chapter presents an alternative formulation of the heterotic string which replaces the internal bosonic degrees of freedom compactified on a 16-dimensional torus by 32 world sheet fermionic degrees of freedom, a process referred to as fermionization. It illustrates fermionization (and the inverse process of bosonization) by considering the simplest case of a single bosonic degree of freedom compactified on a circle.