In the early 1980s estimates of how many species there are on Earth increased by several orders of magnitude; from 2 to 3 million to more than 100 million. Since that time much effort has been placed on refining such species estimates, particularly for insects. Estimates for other non-vertebrate groups and for non-vascular plants also vary enormously and often lack real scientific rationale. A more realistic estimate for all species on Earth is about 10 to 15 million species. This paper examines some of the ecological and systematic constraints on clarifying the magnitude of global diversity.