Permanent microscope slide mounts have been an integral part of the Natural History Museum's (NHM) collections since microscopes were first used in the study of natural history specimens. This paper discusses the methods, materials and mountants used in the preparation of such slides and their storage based on the findings of a museum-wide survey. Over the years, many recipes for slide mounting media have been tried, the mountants differing in their optical properties and their ageing characteristics. Using the collections as a data base of deterioration dating back to the early 19th century, conclusions are drawn as to which are the most suitable mounting media and which are the most suitable conservation techniques and housing methods. Appended are a list of recipes and a full bibliography.