We present models of the spectral energy distributions for an evolutionary sequence of young stellar objects, from protostars to pre-main-sequence stars. The protostellar theory, characterized by a central star and disk embedded within an infalling envelope of dust and gas, can explain the observed infrared spectra of embedded IRAS sources associated with molecular cloud cores. Next, we find that T Tauri stars with near- and mid-infrared excesses can be understood as young stars surrounded by nebular disks. The disks in T Tauri systems are found in two varieties: passive disks which have no intrinsic luminosity and merely intercept and re-radiate stellar photons, and active disks which have appreciable intrinsic luminosity. In addition, there is another class of objects in which a residual dust shell still surrounds the star and disk.