The companion to this book, "Protein structure: a practical approach", shows methods of determining the structure of a newly discovered protein, but that is only part of the story. This text completes the picture by providing the techniques for elucidating the functional properties of proteins, which can vary enormously, from simple structural roles to catalysis, from information transmission to energy control. This collection of tried-and-tested protocols will be applicable to many proteins, and can be performed in an average laboratory, without the need for specialized equipment.