A general discussion of the electronic band structure properties of YBa₂Cu₃O_7-x and other (1,2,3) high-temperature superconductors is presented. Both the occupied and unoccupied density of states are discussed. The key experimental results and their relation to proposed superconducting mechanisms are examined. The discovery of high-temperature superconductors has evoked several proposed mechanisms. Among the most important properties that bear on the proposed mechanism is the material electronic structure. As a result, several experiments probing the occupied and unoccupied density of states have been performed on YBa₂Cu₃O_7-x and other (1,2,3) high-temperature superconductors. The present article provides a critical overview of the results as they bear on the proposed mechanism. After outlining the pertinent theoretical issues, each experimental technique and the results obtained will be discussed