It is a pleasure to introduce Volume 5 in the Methods in Pharmacology series. In 1971, Volume 1 of this series was published while I was Head of the Division of Myocardial Biology in the Department of Pharmacology at Baylor College of Medicine in Houston, Texas. I dedicated that first volume to Sir Henry Hallet Dale, who died on July 23, 1968. In the Preface I pointed out that many of the pharmacological advancements that occurred during the last century were direct descendants from the classic paper published in 1910 by Professor Dale and his colleague, Dr. Barger. In this paper, the concept of "specific receptor sites" was introduced by the statement that "the relationship of the recepted mechanism to the base [i. e. , drug base] may well be one of solid solution of adsorption and, therefore, more analogous to that of an enzyme to its substrate . . . . " I also pointed out at that time that the search for drug receptors continues and that fundamental knowledge of the nature of receptors and drug-receptor interaction will eventually lead to a rational approach to drug design. Since 1971, the study of receptors and their interaction with specific chemical substances has continued at an accelerated pace and this is due, in particular, to the introduction of new and exciting methodologies. The death last year of Professor Raymond P. Ahl­ quist, who pioneered the introduction of specific adrenergic receptors, represents the close of yet another era.