We investigate the low-energy phenomenology of string models constructed in the fermionic formulation. Models are constructed in which the gauge group SU(3){times}SU(2){times}U(1){times}... is obtained directly from free world-sheet fermions. We also construct models in which SU(3){times}SU(2){times}U(1){times}... is obtained by symmetry breaking along completely flat directions in the effective potential; this corresponds to world-sheet interactions. In this category we construct models in which SU(5) is broken to SU(3){times}SU(2){times}U(1) by a Higgs boson in the adjoint representation and in which SO(4){times}SO(6) is broken by the spinor representation. We analyze the effects of the spin structure, massless moduli, and Fayet-Iliopoulos {ital D} terms on low-energy phenomenology. In particular we consider the renormalization-group flow of the couplings, proton decay, mass matrices, Higgs-boson mass, and charge quantization. We briefly compare these results with the flipped SU(5) approach and other constructions such as orbifolds and Calabi-Yau manifolds.