  We study linear actions of algebraic groups on smooth projective varieties X.
A guiding goal for us is to understand the cohomology of "quotients" under such
actions, by generalizing (from reductive to non-reductive group actions)
existing methods involving Mumford's geometric invariant theory (GIT). We
concentrate on actions of unipotent groups H, and define sets of stable points
X^s and semistable points X^{ss}, often explicitly computable via the methods
of reductive GIT, which reduce to the standard definitions due to Mumford in
the case of reductive actions. We compare these with definitions in the
literature. Results include (1) a geometric criterion determining whether or
not a ring of invariants is finitely generated, (2) the existence of a
geometric quotient of X^s, and (3) the existence of a canonical "enveloping
quotient" variety of X^{ss}, denoted X//H, which (4) has a projective
completion given by a reductive GIT quotient and (5) is itself projective and
isomorphic to Proj(k[X]^H) when k[X]^H is finitely generated.
