We present the coefficient of the chromomagnetic interaction operator,
the only unknown coefficient in the Heavy Quark Effective Theory
(HQET) lagrangian up to the $1/m$ level,
with the two–loop accuracy by matching scattering amplitudes
of an on–shell heavy quark in an external field in full QCD and HQET,
and obtain the two–loop anomalous dimension of this operator in HQET.