  The effect of Cooperons on metal-insulator transitions (MIT) in disordered
interacting electronic systems is studied. It is argued that previous results
which concluded that Cooperons are qualitatively unimportant near the MIT might
not be correct, and that the problem is much more complicated than had
previously been realized. Although we do not completely solve the Cooperon
problem, we propose a new approach that is at least internally consistent.
Within this approach we find that in all universality classes where Cooperons
are present, i.e. in the absence of magnetic impurities and magnetic fields,
there are logarithmic corrections to scaling at the MIT. This result is used
for a possible resolution of the so-called exponent puzzle for the conductivity
near the MIT. A discussion of the relationship between theory and experiment is
given. We also make a number of predictions concerning crossover effects which
occur when a magnetic field is applied to a system with Cooperons.
