The Farlow Herbarium houses approximately 1,400,000 specimens, including approximately 75,000 types, of lichenized and non-lichenized fungi, bryophytes, diatoms and algae. The collections are worldwide in scope, with particular strengths in bryophytes and fungi from Asia, Antarctic lichens, entomogenous fungi, and herbaria containing type specimens and bound, indexed exsiccatae of special authors.  

Among the most important collections are those of:

E. Bartram (Mosses), E. A. Burt (Fungi), C. A. Cheever (Diatoms), M. A. Curtis (Fungi), C. W. Dodge (Lichens), W. G. Farlow (Fungi), M. Fleischer (Mosses), N. Patouillard (Fungi), V. Schiffner (Mosses and Hepatics), W. S. Sullivant (Mosses and Hepatics), T. Taylor (Mosses, Hepatics, Fungi and Lichens), R. Thaxter (Fungi, Laboulbeniales) and E. Tuckerman (Lichens).

The diatom collection of about 30,000 microscope slides and field samples documents the work of the international community of diatomists during the nineteenth and twentieth centuries, especially of the period from 1830 to about 1940. A catalog of these collections is available online.