This volume has evolved from a laboratory methods book that one of us first compiled nearly fifteen years ago. Since that time the book has undergone many minor revisions in order to include new methods and updated versions of older methods. The result has been an increasingly useful and more widely circulated book. However, the recent series of technological explosions generally lumped together under the name of "recombinant DNA technology" has been a turning point in the evolution of this previously underground publication. Minor revisions will no longer do. To keep the book useful we have had to make major revi­ sions and additions. The result is a dramatically expanded book that should be more useful to more people. The larger size and wider usefulness of the book have made this more formal publication seem a reasonable step to take. One of the reasons that this volume should be useful to many people is that it includes only procedures that have been used repeatedly by us and that have proven highly reliable both to ourselves and to others in our laboratories.