  We review our current understanding of the progenitors of both long and short
duration gamma-ray bursts (GRBs). Constraints can be derived from multiple
directions, and we use three distinct strands; i) direct observations of GRBs
and their host galaxies, ii) parameters derived from modeling, both via
population synthesis and direct numerical simulation and iii) our understanding
of plausible analog progenitor systems observed in the local Universe. From
these joint constraints, we describe the likely routes that can drive massive
stars to the creation of long GRBs, and our best estimates of the scenarios
that can create compact object binaries which will ultimately form short GRBs,
as well as the associated rates of both long and short GRBs. We further discuss
how different the progenitors may be in the case of black hole engine or
millisecond-magnetar models for the production of GRBs, and how central engines
may provide a unifying theme between many classes of extremely luminous
transient, from luminous and super-luminous supernovae to long and short GRBs.
