A herbarium (pl.: herbaria) is a collection of preserved plant specimens and associated data used for scientific study.

The specimens may be whole plants or plant parts; these will usually be in dried form mounted on a sheet of paper (called exsiccatum, plur. exsiccata) but, depending upon the material, may also be stored in boxes or kept in alcohol or other preservative. The specimens in a herbarium are often used as reference material in describing plant taxa. Some specimens may be types, some may be specimens distributed in published series called exsiccatae.

The term herbarium is often used in mycology to describe an equivalent collection of preserved fungi, otherwise known as a fungarium. A xylarium is a herbarium specialising in specimens of wood. The term hortorium (as in the Liberty Hyde Bailey Hortorium) has occasionally been applied to a herbarium specialising in preserving material of horticultural origin.

The techniques for making herbaria have changed little over at least six centuries. They have been an important step in the transformation of the study of plants from a branch of medicine to an independent discipline, and to make available plant material from far away places and over a long period of time.

The oldest traditions of making herbarium collections have been traced to Italy. The Bologna physician and botanist, Luca Ghini (1490–1556) reintroduced the study of actual plants as opposed to relying on classical texts, such as Dioscorides, which lacked sufficient accuracy for identification. At first, he needed to make available plant material, even in winter, hence his Hortus hiemalis (winter garden) or Hortus siccus (dry garden). He and his students placed freshly gathered plants between two sheets of paper and applied pressure to flatten them and absorb moisture. The dried specimen was then glued onto a page in a book and annotated. This practice was supplemented by the parallel development of the Hortus simplicium or Orto botanico (botanical garden) to supply material, which he established at the University of Pisa in 1544.

Although Ghini's herbarium has not survived, the oldest extant herbarium is that of Gherardo Cibo from around 1532. and in the Lower Countries the hortus siccus (1566) of Petrus Cadé. While most of the early herbaria were prepared with sheets bound into books, Carl Linnaeus came up with the idea of maintaining them on free sheets that allowed their easy re-ordering within cabinets.

Commensurate with the need to identify the specimen, it is essential to include in a herbarium sheet as much of the plant as possible (e.g., roots, flowers, stems, leaves, seed, and fruit), or at least representative parts of them in the case of large specimens. To preserve their form and colour, plants collected in the field are carefully arranged and spread flat between thin sheets, known as flimsies (equivalent to sheets of newsprint), and dried, usually in a plant press, between blotters or absorbent paper.

During the drying process the specimens are retained within their flimsies at all times to minimize damage, and only the thicker, absorbent drying sheets are replaced. For some plants it may prove helpful to allow the fresh specimen to wilt slightly before being arranged for the press. An opportunity to check, rearrange and further lay out the specimen to best reveal the required features of the plant occurs when the damp absorbent sheets are changed during the drying/pressing process.^{[citation needed]}