A radioligand is simply a radioactively labeled ligand, and is often used to study binding characteristics of receptors, such as receptor density, distribution, or affinity in the lab. A radiotracer, on the other hand, is a radioactive compound used in a wider range of scientific and medical applications, such as imaging studies and disease diagnosis. While radioligands focus on receptor interactions at the molecular level, radiotracers are used for measuring biochemical processes in the body.