Large-Scale simulation modeling has recently emerged as a widely supported approach to policy evaluation in population and development. More than a dozen large computer models, multipurpose, disaggregated, and highly endogenous, now exist; others are being built. This paper examines the thinking behind these models and assesses their potential use to development planners. The model builders claim to provide a valuable tool for policy evaluation by tracing out both qualitatively and numerically the long-term implications of policy decisions. This claim is disputed. Specific faults identified include inappropriate accounting structures, over simplyfying of issues behind a superficial complexity, and a lack of concern with validation. Smaller, issue-specific models are preferred.