The first decade of oxide superconductor processing has taught us that processing purely by empirical means is unsatisfactory. The materials chemistry and thermodynamics of the REBa2Cu3O7-x (RE = rare earth) series are reviewed, and examples of how the results can lead to advances in processing are presented. Particular attention is paid to phase stability and its relationship to various processing methods, including single-crystal growth and thin-film routes. Future areas of research are highlighted.