This chapter begins with a brief introduction by Rogers, in which she highlights the continued salience of Carol Weiss's decade-old questions about theory-based evaluation. Theory-based evaluation has developed significantly since Carol Weiss's chapter was first published ten years ago. In 1997 Weiss pointed to theory-based evaluation being mostly used in the areas of health promotion and risk prevention. The use of program theory is now commonplace and Weiss' article has been cited in a wide range of program areas, including evaluations of energy conservation, comprehensive community-based initiatives, supported housing, gaming and simulation, and anticorruption activities. Weiss' article remains highly relevant because of her warnings about the traps and challenges in using program theory. A reprint of Weiss's 1997 article concludes this chapter. (Contains 6 figures.)