This article discusses the rapidly emerging field of computer-based assessment for adaptive content in e-learning (National Research Council, 2001), which we call differentiated e-learning. In e-learning products, a variety of assessment approaches are being used for such diverse purposes as adaptive delivery of content, individualizing learning materials, dynamic feedback, cognitive diagnosis, score reporting, and course placement (Gifford, 2001). A recent paper at the General Teaching Council Conference in London, England, on teaching, learning, and accountability described assessment for personalized learning through e-learning products as a “quiet revolution” taking place in education (Hopkins, 2004). In our study, we examine approaches for the use of assessment evidence in e-learning in four case studies. The products in the case studies were selected for exhibiting at least one exemplary aspect regarding assessment and measurement. The principles of the Berkeley Evaluation & Assessment Research Center Assessment System (Wilson & Sloane, 2000) are used as a framework of analysis for these products with respect to key measurement principles.