The objective of this chapter is to update the current understanding of the unique role of eastern old-growth forests in supporting biological diversity, a topic that was first addressed for vertebrates and understory herbs by various contributors to Davis (1996). Therefore, this treatment will focus on diversity of invertebrates, bryophytes, lichens, fungi, vascular plants, and their respective interactions. The biological significance of eastern old-growth forests is linked, in large part, to heterogeneous habitats provided by dead trees (snags), rotting logs, and old live trees. Therefore, it is necessary to first describe eastern old-growth forest structure and contrast it with that of variously disturbed and managed forests in order to establish context for the ecological significance of the structure-diversity relationships.