  I review aspects of the theory of long-duration gamma-ray burst (GRB) central
engines. I focus on the requirements of any model; these include the angular
momentum of the progenitor, the power, Lorentz factor, asymmetry, and duration
of the flow, and both the association and the non-association with bright
supernovae. I compare and contrast the collapsar and millisecond proto-magnetar
models in light of these requirements. The ability of the latter model to
produce a flow with Lorentz factor ~100 while simultaneously maintaining a
kinetic luminosity of ~10^50 ergs/s for a timescale of ~10-100 s is emphasized.
