  We calculate the decay $\eta\to3\pi$ at next-to-next-to-leading order or
order $p^6$ in Chiral Perturbation Theory. The corrections are somewhat larger
than was indicated by dispersive estimates. We present numerical results for
the Dalitz plot parameters, the ratio $r$ of the neutral to charged decay and
the total decay rate. In addition we derive an inequality between the slope
parameters of the charged and neutral decay. The experimental charged decay
rate leads to central values for the isospin breaking quantities $R=42.2$ and
$Q=23.2$.
