  Static spherically symmetric perfect fluid solutions are studied in metric
$f(R)$ theories of gravity. We show that pressure and density do not uniquely
determine $f(R)$ ie. given a matter distribution and an equation state, one
cannot determine the functional form of $f(R)$. However, we also show that
matching the outside Schwarzschild-de Sitter-metric to the metric inside the
mass distribution leads to additional constraints that severely limit the
allowed fluid configurations.
