Adult basic education programs, sometimes called adult basic and secondary education programs, typically serve adults over the age of sixteen who do not have a high school diploma and are no longer eligible for traditional secondary education programs. Although adult basic education (ABE) is situated apart from the elementary, secondary, and college education systems, it does not exist in a vacuum. This is especially true of literacy assessment in adult basic education now, at the turn of the century. Adult literacy assessment is affected by changing definitions of literacy, changes in the needs of federal and state funders, and changes in assessment tools and practices. These changes are discussed in detail in this chapter in order to present a broad picture of the current state of literacy assessment in adult basic education. Because none of these changes are completely new, a brief history is presented to put them in context. Literacy must be defined before it is possible to know how to assess it, and assessment must be defined before it is possible to know how best to implement it. This chapter thus begins with definitions of literacy and then describes assessment in adult basic education within this definitional framework. Implications for practice, research, and policy follow. (Contains 1 note.) [Chapter 4 in: Annual Review of Adult Learning and Literacy. Volume 3; copublished with John Wiley & Sons, Inc.; see ED478329.]