  We review a method to find the non-abelian open superstring effective action,
thereby settling the issue of the ordering ambiguities. We start from solutions
to Yang-Mills which, in D-brane context, define certain BPS configurations.
Studying their deformations in the abelian case shows that the Born-Infeld
action is the unique deformation which admits solutions of this type. By
applying the method to the non-abelian case we calculated the full effective
action through O(\alpha'^3). The presence of derivative terms turns out to be
essential. Testing the result by comparing the spectrum in the presence of a
constant magnetic background field with the string theory prediction, we obtain
perfect agreement.
