The growth of natural history collections in America, and the Harvard University Herbaria in particular, owe much to the early days of exploration. In successive waves of exploration of the expanding territory known as the American West, private collectors accompanying various survey expeditions sent back to scholars in the east samples of the plants they encountered. In the Boston area, Asa Gray worked to describe and catalogue these collections, many of which were new to science. At the same time, Gray's exchange of specimens with his colleagues in other parts of the world and expansion of exploration in the Old World resulted in the growth of a collection that was both cosmopolitan in scope and unique in much of its representation.

The rich and varied Harvard University botanical collections can all be traced back to those activities of Asa Gray. Gray came to Harvard in 1842, before which there were few, if any, botanical collections at Harvard, although the Botanical Garden was founded earlier, in 1807, and some instruction in plant science had been given before Gray's arrival. Gray's own activities in teaching and in research led to the training of many American botanists, to Gray's recognition as an international figure in biology, and to the founding of the Gray Herbarium. Several of his students and associates also came to found or develop separate botanical institutions at Harvard. These were: Charles Sprague Sargent, first director of the Arnold Arboretum; George Lincoln Goodale, first director of the Botanical Museum; and William Gilson Farlow, founder of the Farlow Reference Library and Herbarium of Cryptogamic Botany.

These units with their areas of specialization have been the centers of botanical systematics and related research at Harvard. Vascular plants are the emphasis of the Gray Herbarium (GH), the Herbarium of the Arnold Arboretum (A), the Economic Herbarium of Oakes Ames (ECON), and Oakes Ames Orchid Herbarium (AMES); non-vascular plants and fungi are the purview of the Farlow Herbarium (FH) collections. Each developed a library rich in systematic treatments as well as special subjects such as economic botany, history of botany, travel and exploration, ethnobotany, orchidology, forestry, etc.

Through field trips, gifts, exchanges, and purchases, the collections of these institutions grew. Until the early 1950's three of these collections were housed separately in Cambridge and one was at the Arnold Arboretum in the Jamaica Plain section of Boston. In Cambridge, the Gray Herbarium was located on Garden Street, within the Botanical Gardens, but blocks from the Biological Laboratories. The Farlow Herbarium was located adjacent to the Biological Laboratories on Divinity Avenue and across a courtyard from the Botanical Museum. Upon construction of a new herbarium building on Divinity Avenue in 1954, the collections of the Gray Herbarium and Arnold Arboretum were brought together (except for cultivated plants of the Arnold Arboretum which remained in Jamaica Plain) and integrated, as were the libraries of those two institutions. The Arnold, Gray, Farlow and the Botanical Museum herbaria were thus close at hand.

Despite their contiguity, each unit was separately administered until the mid-1970's when administrative functions began to be merged. The vascular plant collections of A, AMES, ECON, GH are now integrated. The New England Botanical Club Herbarium (NEBC), is in the process of being integrated with the Harvard collections. Although the specimens are now integrated, the institution to which a specimen belongs is stamped on each sheet. The acronym for each herbarium should be indicated when citing Harvard specimens.