Dr. Charles W. Reimer dedicated almost 50 years of his life to the development and maintenance of the ANSP Diatom Herbarium. Today this is one of the largest diatom collections in the World. Its holdings include many historic and taxonomically important collections of prominent diatomists. Extensive collections of diatoms from river, lake, and wetland surveys conducted across North America by ANSP staff, environmental agencies, corporations, and individual researchers represent the core of the Herbarium holdings, which are an important resource for diatom systematic, biogeographical and environmental studies. This paper gives an overview of the history of the Herbarium, its holdings and structure, and ongoing renovations of the collection.