  This paper shows that the satisfiability problem for probabilistic CTL (PCTL,
for short) is undecidable. By a reduction from $1\frac{1}{2}$-player games with
PCTL winning objectives, we establish that the PCTL satisfiability problem is
${\Sigma}_1^1$-hard. We present an exponential-time algorithm for the
satisfiability of a bounded, negation-closed fragment of PCTL, and show that
the satisfiability problem for this fragment is EXPTIME-hard.
