The scientist' s understanding of the cell at the molecular level has advanced rapidly over the last twenty years. This improved understa- ing has led to the development of many new laboratory methods that increasingly allow old problems to be tackled in new ways. Thus the modern scientist cannot specialize in just one field of knowledge, but must be aware of many disciplines. To aid the process of investigation, the Methods Molecular Biology series has brought together many protocols and has highlighted the useful variations and the pitfalls of the different methods. However, protocols frequently cannot be simply taken from the shelf. Thus the starting sample for a chosen protocol may be unavailable in the correct state or form, or the products of the procedure require a different sort of processing. Therefore the scientist needs more detailed information on the nature and requirements of the enzymes being used. This information, though usually available in the literature, is often widely dispersed and frequently occurs in older volumes of journals; not everyone has comprehensive library facilities available. Also many scientists searching out such information are not trained enzymologists and may be unaware of some of the parameters that are important in a specific enzyme reaction.