Data exchange is the process of taking ata structured under a source schema and actually transforming it into data structured under a target schema, so that the target data is an accurate representation of the source data. Data exchange is similar to the related concept of data integration except that data is actually restructured (with possible loss of content) in data exchange. There may be no way to transform an instance given all of our constraints. Conversely, there may be numerous ways to transform the instance (possibly infinitely many), in which case we must identify and justify a "best" choice of solutions.