The discovery of the first bona fide brown dwarfs in 1995 ushered in a new era in both stellar and planetary astrophysics. The emergent spectra of brown dwarfs are distinct from those of the lowest-mass stars and thus the creation of three new spectral classes, L, T, and Y, were required in order to classify them. In this chapter, I provide a historical review of the creation of these spectral classes and briefly discuss the physical conditions that give rise to the variations in spectral morphology used for classification.