  We show that the number $g_n$ of labelled series-parallel graphs on $n$
vertices is asymptotically $g_n \sim g\cdot n^{-5/2} \gamma^n n!$, where
$\gamma$ and $g$ are explicit computable constants. We show that the number of
edges in random series-parallel graphs is asymptotically normal with linear
mean and variance, and that the number of edges is sharply concentrated around
its expected value. Similar results are proved for labelled outerplanar graphs
and for graphs not containing $K_{2,3}$ as a minor.
