This, the 148th volume of the Handbook of Experimental Pharmacology series, focuses on the very core of pharmacology, namely receptor theory. It is fitting that the originator of receptor pharmacology, A. J. CLARK, authored the fourth volume of this series 63 years ago. In that volume CLARK further developed his version of receptor theory first described four years earlier in his classic book The Mode of Action of Drugs. An examination of the topics covered in volume 4 reveals a striking similarity to the topics covered in this present volume; pharmacologists today are still as interested in...