Single crystal scintillators of Lu2SiO5:Ce (LSO:Ce) were first developed about 15 years ago and have been in commercial use in positron emission tomography systems for more than five years. Annual production now exceeds 10,000 kg. We review the development of LSO:Ce and its implementation in PET, and summarize the current understanding of the scintillation characteristics and mechanisms. In addition, we present the evolution of LSO:Ce detector design and consider the potential performance of future imaging systems. In particular, coincidence timing measurements are presented and time-of-flight systems that take better advantage of the timing characteristics of LSO:Ce are discussed.