Adenosine is usually the product of the hydrolysis of extracellular adenosine triphosphate (ATP) and is widely distributed in the human body. It causes or modulates a variety of functions, including cell growth, heart rhythm, immune function, sleep regulation, and angiogenesis. Adenosine acts by activating specific G protein-coupled receptors (GPCRs), which are divided into four subtypes: the adenosine 1 receptor (A1R), adenosine 2A receptor (A2AR), adenosine 2B receptor (A2BR), and adenosine 3 receptor (A3R). Extensive investigations over the past 40 years have identified numerous effects of the adenosine receptors as shown below, identifying them as potential drug targets for treating numerous diseases.