This paper presents concepts and principles pertinent to the development of cross-level and multilevel theory in organizational science by addressing a number of fundamental theoretical issues. It describes hierarchy theory, systems theory, and mixed-level models of organization developed by organizational scientists. Hierarchy theory derives from the notion that complex systems exhibit hierarchical structure--a pattern of relations among levels--and offer a framework for cross-level predictions of organizational processes and activities. Systems theory describes similar characteristics of structure, process, and function shared by different entities. It can be used to derive organizationally relevant generalizations that include isomorphisms and processes operating at different echelons. Mixed-level models include composition models, cross-level models, and multilevel models. Issues of level in organizational research are a new frontier and expand the model of organizational science by integrating research areas and providing specific models of organizational behavior. An extensive bibliography is included. (MD)