The gamma-ray energy tracking array GRETA marks a major advance in the development of gamma-ray detector systems and can provide order-of-magnitude gains in sensitivity compared to existing arrays. It uses highly segmented hyper-pure germanium crystals together with advanced signal processing techniques to determine the location and energy of individual γ-ray interactions, which are then combined to reconstruct the incident γ-ray in a process called tracking. The GRETA science program described in this document illustrates GRETA’s broad impact in many experimental programs in nuclear structure and nuclear astrophysics, and its central role in the Facility for Rare Isotope Beams, FRIB, being constructed at Michigan State University.