  We use Lorentz invariance and the QCD equations of motion to study the
evolution of functions that appear at leading (zeroth) order in a 1/Q expansion
in azimuthal asymmetries. This includes the evolution equation of the Collins
fragmentation function. The moments of these functions are matrix elements of
known twist two and twist three operators. We present the evolution in the
large N_c limit, restricted to the non-singlet case for the chiral-even
functions.
