Notions of hierarchical structure or levels of organization play a number of important theoretical roles in the biological sciences, and can be found operating in the background of several recent debates in philosophy of science. Nevertheless, apart from some local exceptions, these notions have not received the careful scrutiny they deserve. We propose to make them the subject of an Altenberg Workshop. Our proposed workshop will provide a venue for leading contributors on the topic to engage in a collective re-examination of the concept of levels of organization as it arises in specific problem-contexts. These include questions about how to rigorously formulate a conception of levels, hierarchical evolutionary theory, structural and ecological hierarchy theories, issues surrounding levels and causation, multilevel explanation, and finally, critical challenges to the usefulness of hierarchy and levels.