ClassSheets are a high level, object-oriented modeling language for spreadsheets. Integrating concepts from the Unified Modeling Language (UML), this language provides a modular and abstract methodology for dealing with spreadsheets, and namely to specify and maintain their business logic. This methodology envisions concrete spreadsheets (or spreadsheet instances) being automatically derived from, and maintained together with abstract specifications (or spreadsheet models) [5], [6]. This environment provides an efficient and effective model-driven spreadsheet development system. Indeed, errors can be prevented by carefully reasoning about, and designing, a concise model,instead of doing so with potentially large spreadsheets.