Photoemission studies generally give information on the electronic structure of a material, not the geometrical structure, although some surface structural information can be obtained by photoelectron spectroscopy. Knowledge of the crystal structures of the high-Tc materials is important in photoemission studies in several ways. First, crystal structures are required to determine the crystal potential for the calculation of the electronic structures. Even if a simple model of the crystal is being used, as in cluster calculations, the model should resemble part of the full crystal structure. Second, angle-resolved photoemission provides information in reciprocal space: energies as a function of angle or wave vector. The Bravais lattice of the crystal determines the Brillouin zone, hence the “space” in which theory and experiment often are compared. Third, the amount and nature of the anisotropy of the crystal structure, although difficult to quantify, are helpful in determining the anisotropy one might expect in physical properties, including photoelectron spectra. Orienting single crystals for angle-resolved photoemission studies also requires some structural knowledge. Fourth, there is an important experimental consideration. Most photoelectron spectroscopy is carried out on clean surfaces produced by cleaving in ultrahigh vacuum. It often is crucial to know what the cleavage surface is. Core-level photoelectron spectroscopy can help identify the atomic character of a cleaved surface, but knowledge of the crystal structure is needed and some idea of interplanar bonding is helpful. Fortunately, a structure determination is usually one of the first studies made on any new material.