With the recent purification and cloning of the A1AR and the cloning of the A2AR in association with the development of selective radioligands, we are now poised to begin to understand at the most fundamental level the structure, function, and regulation of adenosine receptors. Although adenosine's physiological effects have been appreciated for more than 60 years, we are only now ready to address questions at the biochemical and molecular biological levels. We are likely to begin to see evidence for a whole group of adenosine receptors undetected by previous technology. One era of adenosine receptor research has just ended, and we now enter the new with anxious anticipation.