The herbarium at NIU is an internationally recognized systematic collection, initiated in the 1960s and enlarged significantly during the 1970s to the present. Holdings presently total about 23,000 specimens of vascular plants plus an additional 8000 bryophytes. A very large portion of the bryophytes (about 6000 specimens) is on permanent loan to MO (Missouri Botanic Garden).

Special collections include a definitive collection of the genus Dahlia, which contains holotypes and isotypes plus a thoroughly documented array of hybrids between authenticated wild species. A second important collection is of Arbutus that also includes type material. The troublesome genus Agrimonia is more completely documented at DEK than at any other herbarium in the world. In Zygophyllaceae, there is a substantial collection of species of Guaiacum, for which sequenced marker loci are indicated. The collection also emphasizes grassland species and those for which genetic resources, especially complete plastome sequences, are available.

DEK presently maintains exchange relationships with more than 26 domestic and 6 foreign herbaria. As a consequence, coverage, while emphasizing the flora of Illinois, is worldwide.