Like any infrastructure technology, Virtual World (VW) platforms provide affordances that facilitate some activities and hinder others. Although it is theoretically possible for a VW platform to support all types of activities, designers make choices that lead technologies to be more or less suited for different learning objectives. Virtual World platforms' capabilities can be characterized in terms of the extent to which they are multiple or special purpose and the degree to which they support incorporation of few or many knowledge resources. Matching these capabilities with a framework for characterizing instructional approach and learning objectives provides a basis for selecting, piloting, and advocating use of particular VW platforms in specific educational contexts. (Contains 4 figures and 3 tables.)