These unique constraints include short synthesis (often limited to20r3 half-lives) times and control from behind bulky shielding structures that make both access to, and visibility of, radiochemical processes and equipment difficult. Often Curie levels of positron emitting nuclides are required for synthesis of PET radiopharmaceuticals, making this potentially dangerous for a radiochemist or laboratory specialist. The use of short half-lived radionuclides also necessitates that many PET radiotracers (particularly those labeled with 1lC, *3N, and '50) be synthesized repetitively during the day, each dose being produced separately just before administration.Radiotracer synthesis must be reliable and efficient to keep the costs of PET procedures down. Furthermore, radiotracer synthesis procedures for human use must produce pharmaceutical quality products and be well documented and controlled to help satisfy requirements of federal and local regulations on human research.Automation can help PET research institutions overcome all of these potential '" limitations. A look at the history of the development of successful automated PET radiotracer synthesis machines reveals a richness in engineering solutions to these problems that still exists today.