  The rare decays $B^0 \to K^{*0} \mu^+ \mu^-$ and $B_s \to \phi \mu^+ \mu^-$
are now being observed with enough precision to test Standard Model
predictions. A full understanding of these decays requires accurate
determinations of the corresponding hadronic form factors. Here we present
results of lattice QCD calculations of the $B \to K^*$ and $B_s \to \phi$ form
factors; we also determine the form factors relevant for the tree-level decays
$B_s \to K^* \ell \nu$. We use full-QCD configurations including 2+1 flavors of
sea quarks using an improved staggered action, and we employ lattice
non-relativistic QCD to describe the bottom quark.
