This chapter explains the construction of the text-task-respondent (TTR) theory of functional literacy and its theoretical and mathematical models. The elements of functional literacy discussed in earlier chapters-the prose, document, and quantitative literacy task demands, the seven sets of corresponding literacy skills, and the text features-are discussed within the context of the theory. Included is information about the use of multidimensional item response theory (IRT) modeling to empirically validate the theory.