  How massive were the first stars? This question is of fundamental importance
for galaxy formation and cosmic reionization. Here we consider how protostellar
feedback can limit the mass of a forming star. For this we must understand the
rate at which primordial protostars accrete, how they and their feedback output
evolve, and how this feedback interacts with the infalling matter. We describe
the accretion rate with an ``isentropic accretion'' model: the rate is
initially very large (~0.03 M_sun/yr when m_* =1 M_sun) and declines as
m_*^{-3/7}. Protostellar evolution is treated with a model that tracks the
total energy of the star. A key difference compared to previous studies is
allowance for rotation of the infalling envelope. This leads to photospheric
conditions at the star and dramatic differences in the feedback. Two feedback
mechanisms are considered: HII region breakout and radiation pressure from
Lyman-alpha and FUV photons. Radiation pressure appears to be the dominant
mechanism for suppressing infall, becoming dynamically important around 20
M_sun.
