This paper examines the role of species level studies in assessing arthropod biodiversity and understanding the processes that shape it. Examples from one of the most species-rich groups of insects , the parasitic Hymenoptera, are used to examine the importance of data at the species level for understanding faunal patterns, population and community studies, documenting morphological variation, revealing the presence of cryptic species, and supporting and/or testing monitoring procedures. The possible sources of error in identifying species and approaches that will minimize these are discussed.