In this chapter, we discuss the nature of measurement in the social sciences and focus on the concept of isomorphism. Isomorphism is the concept that describes the degree of fit between measuring instruments and the variable being measured; hence, the degree of isomorphism indicates how appropriate the measuring instrument is to the phenomenon being measured. We then present the four levels of measurement-nominal, ordinal, interval, and ratio-and data transformation, the process enabling data to be measured at more than one level. Next we discuss the issue of measurement error. The chapter concludes with descriptions of the concepts of validity-whether an instrument measures what it is supposed to measure-and reliability-whether and by how much measurements are consistent from one observation to the next.