For the past three centuries, scientists have documented the earth's plant and fungal diversity through dried reference specimens maintained in collections known as herbaria. There are approximately 3,000 herbaria in the world today, with approximately 12,000 associated curators and biodiversity specialists. Collectively the world's herbaria contain an estimated 350,000,000 specimens that document the earth's vegetation for the past 400 years. Index Herbariorum is a guide to this crucial resource for biodiversity science and conservation. The Index Herbariorum (IH) entry for an herbarium includes its physical location, URL, contents (e.g., number and type of specimens), founding date, as well as names, contact information and areas of expertise of associated staff. Only those collections that are permanent scientific repositories are included in IH. New registrants must demonstrate that their collection is accessible to scientists, and is actively managed. Each institution is assigned a permanent unique identifier in the form of a one to eight letter code, a practice that dates from the founding of IH in 1935.