  We present a theoretical prediction for the photon spectrum in radiative
Upsilon decay. Parts of the spectrum have already been understood, but an
understanding of the endpoint region has remained elusive. In this paper we
provide the missing piece, and resolve a controversy in the literature. We
treat the endpoint region of Upsilon -> X gamma decay within the framework of
the soft-collinear effective theory (SCET). Within this approach the Upsilon
structure function arises naturally, and kinematic logarithms are summed by
running operators using renormalization group equations. In a previous paper we
studied the color-octet contribution to the decay. Here we treat the
color-singlet contribution. We combine our result with previous results to
obtain the Upsilon -> X gamma spectrum. We find that resumming the
color-singlet contribution in the endpoint gives a result that is in much
better agreement with the data than the leading order prediction.
