Time-of-Flight (TOF) positron emission tomographs (PET) were developed during the 1980s and early 1990s. Initial investigations into the potential improvements in signal-to-noise ratios if TOF information was utilized were published in 1980. By 1982, two groups (Washington University and Commissariat á l'Energie Atomique-Laboratorie d'Electronique et de L'Informatique [CEA-LETI]) were designing and building the first TOF tomographs. A third group at the University of Texas also designed and built a TOF system. These systems were optimized for high count-rate imaging of short-lived radiotracers for applications such as cardiac blood flow. The first system put into operation for patient scans was the Super PETT I built at Washington University by Michel Ter-Pogossian and his colleagues. The Washington University group went on to design two additional versions of TOF systems and the CEA-LETI group developed two basic tomograph designs. As Bismuth Germanate (BGO)-based scanners were refined, it became clear that the TOF systems could not provide the same high spatial resolution as offered by the newer systems. The use of the fast scintillators required for TOF systems also resulted in lower intrinsic sensitivity that was only partially compensated for by the effective gain in sensitivity offered by TOF image reconstruction techniques. Further development of TOF systems was suspended in the early 1990s. With the development of new scintillators that provide more light output and are denser than those available in the 1980s and considerably faster than BGO, there is new interest in the application of TOF techniques for future tomograph designs.