In this chapter, I explore problems of accurate measurement. These are problems that arise when we try to relate the actual operations in a piece of research—that is, measures of things—to the concepts that form the basis of our underlying theory. Concepts, of course, exist only in the mind. One necessary assumption, if we are to claim that a piece of research has tested a given theory, is that the things measured in the research correspond to the things in the theorist’s mind.