The Reaction $\pi N \to \pi \pi N$ at Threshold

I summarize the results of the complete one--loop chiral perturbation theory calculation performed recently. It is shown that it allows to accurately pin down the isospin two, S--wave $\pi \pi$ scattering length $a_0^2$. On the other hand, interesting resonance physics makes a precise determination of $a_0^0$ very difficult.