This chapter identifies the places where the meaning of the data is hidden, and the structure of how changes and mismatches come into being. One of the constants in data processing is that data changes — vales, obviously, but also the attributes and relationships. The chapter explores three categories of change to underlying data: two data sources are merging —s one has information about ‘X’ and the other does not; one data source groups data codes data values one way, and another groups them differently; and there are business rule differences between the data sources. Data may just be strings of 1s and 0s, but the value of that data is the combination of the data and the meaning hidden in the data, data relationships, queries, interpretive programs, and the hidden assumptions of the people who use the data.