for further collapse, but once all of the H$_{2}$ is gone, the collapse becomes fully adiabatic. Detailed simulations of the collapse, using adaptive mesh refinement (AMR; see e.g. @abn02 [@on07]) or smoothed particle hydrodynamics (SPH; see e.g. @brlb04 [@yoha06; @yoh08]) show little or no evidence for fragmentation between $n \sim 10^{4} \: {\rm cm^{-3}}$ and the onset of this adiabatic evolution, and it is natural to associate the latter with the formation of a primordial protostar. The amount of gas incorporated into the protostar at this point is small, approximately $0.01 \: {\rm M_{\odot}}$, but it is surrounded by a massive, dense envelope and hence begins to accrete rapidly. If the envelope does not fragment, and if accretion onto the protostar is unhindered by radiative feedback, then the final mass of the star can be very large. For example, @on07 estimate that its mass could lie anywhere in the range $M \sim 20$–$2000 \: {\rm M_{\odot}}$, depending on the environment and dynamical history of the gas. The degree to which feedback will suppress accretion remains uncertain, but the most effective potential feedback mechanisms, Lyman-$\alpha$ scattering and photoionization [@mt08] become important only once the mass of the star exceeds 20 M$_{\odot}$. 