Instructional design (ID) and human performance technology (HPT) stem from a common origin in systems thinking and behavioral psychology, but today the two fields employ different research bases, system foci, and methods. To contrast these fields, this chapter presents an idealized and abstracted discussion that examines the theoretical origins of the two fields, briefly describes their similarities, and focuses on their differences in terms of analytical frameworks and methods. We conclude that contemporary practice in most contexts combines elements of ID and HPT, particularly when working in cross-functional teams seeking to improve organizational performance. Practitioners of ID are likely to encounter HPT in their work, and they may be called upon to serve as part of a cross-functional team using HPT as a common conceptual framework.