Since their discovery approximately 25 years ago, adenosine receptors have now emerged as important novel molecular targets in disease and drug discovery. These proteins play important roles in the entire spectrum of disease from inflammation to immune suppression. Because of their expression on a number of different cell types and in a number of different organ systems they play important roles in specific diseases, including asthma, rheumatoid arthritis, Parkinson's disease, multiple sclerosis, Alzheimer's disease, heart disease, stroke, cancer, sepsis, and obesity. As a result of intense investigations into understanding the molecular structures and pharmacology of these proteins, new molecules have been synthesized that have high specificity for these proteins and are now entering clinical trials. These molecules will define the next new classes of drugs for a number of diseases with unmet medical needs.