The Department of Defense (DOD) has recently concluded that modeling and simulation will be a fundamental part of its activities in functional areas that include education, training, and operations planning; research and development; production and logistics; test and evaluation; and analysis. All of this will require an unprecedented degree of interoperability among models. In addition, some of the traditional distinctions among classes of activity are beginning to break down as the DOD increasingly emphasizes such approaches as learning and training with the same tools one would use to fight. One consequence of these and related trends is an awakened interest in the capability to vary levels of resolution readily — either within a single model or within a family of related models. This Note introduces and discusses variable-resolution modeling. Specifically, the authors discuss why various levels of resolution are needed in combat models and why many model users have not fully appreciated this need; describe several forms that variable-resolution modeling can take; illustrate the concepts with examples and identify design issues; and present conclusions, especially about areas for further research and types of software that should be developed.