  We study loop corrections in boundary string field theory (BSFT). After
commenting on problems with quantizing the tree level BSFT as an ordinary field
theory, we discuss the tree level coupling to closed strings and define the
loop corrections via factorization in the closed string channel. This
description is weakly coupled in the vicinity of the closed string vacuum. Our
proposal for the one-loop effective action differs in general from computing
the annulus or cylinder partition functions. We also compute the decay rates
and the loop corrections to the tensions of unstable branes in perturbative
string theory.
