Current theories of learning emphasize the importance of learner-centered, active, authentic, environments for meaningful knowledge construction. From this perspective, computerized case-based learning systems afford practice fields for learners to build domain knowledge and problem-solving skills and to support contextualized transfer of knowledge and skills to professional practice. Case-based learning systems are playing a more and more important role in clinical practice disciplines that deal with ill-structured problems, especially in medicine and education. Technology is an efficient means to provide such learning environments. However, the design of computerized case-based learning systems is extremely complex. This article applies the eight design principles for creating computer-based practice fields described by Barab and Duffy (2000) to two case-based learning systems: CAMPUS (Computer-assisted Education in Medicine via Platform-Independent System) and the TPSS (Teacher Problem Solving Skills) series of interactive cases. The authors describe the use of these design principles for practice fields in these two learning systems and summarize evaluation findings based on their implementation. The one practice field design principle that cannot be implemented within a computer-based learning system is to provide an opportunity to negotiate meaning within a community of learners. To achieve this design principle, instructors need to provide opportunities for discourse around the cases so that knowledge can be manipulated, re-constructed, and transferred beyond the computer-based learning environment. (Contains 6 figures.)