In this thesis, we study the anisotropy of step stiffness: an important parameter describing the fluctuations of surface steps within the continuum step model. Using a lattice-gas framework, we derive many practical formulas for the anisotropy of step stiffness on face centered cubic {001} and {111} surfaces. We compare our formulas to experiments on Ag and Cu surfaces and thereby predict the size of nearest-neighbor, next-nearest-neighbor, and three-adatom, non-pairwise "trio" interactions between adatoms. To further corroborate our theory, we perform a series of first-principle calculations of the relevant adatom interactions. We also incorporate our formulas into simulations and model the relaxation of a Ag step initially pinned by surface impurities. Finally, we extend our theory to model Ag steps decorated by C_60 molecules. Together, our work provides a consistent picture of step stiffness anisotropy from an experimental, theoretical, and numerical perspective.