The term "illiterate" that is commonly used to describe someone who cannot read or write is an absolute term, which fails to recognize that those who are deemed "illiterate" are probably in fact "literate" to some degree. The approach taken to define and measure literacy in the International Adult Literacy Survey (IALS) acknowledges the limitation of viewing literacy in a dichotomous way. The IALS assesses literacy proficiency along a continuum. This chapter considers some of the conceptual issues surrounding the definition and measurement of literacy proficiency. In particular, it addresses what the IALS literacy proficiency measure is and what it is not.