  Quarkonia ($J/\psi$, $\psi$', $\Upsilon$) production provides a sensitive
probe of gluon distributions and their modification in nuclei; and is a leading
probe of the hot-dense (deconfined) matter created in high-energy collisions of
heavy ions. We will discuss the current understanding of the production process
and of the cold-nuclear-matter effects that modify this production in nuclei in
the context of recent p+p and p(d)+A quarkonia measurements. Then we will
review the latest results for nucleus-nucleus collisions from RHIC, and
together with the baseline results from d+A and p+p collisions, discuss several
alternative explanations for the observed suppressions and future prospects for
distinguishing these different pictures.
