  The adiabatic motion of a charged, spinning, quantum particle in a two -
dimensional magnetic field is studied. A suitable set of operators generalizing
the cinematical momenta and the guiding center operators of a particle moving
in a homogeneous magnetic field is constructed. This allows us to separate the
two degrees of freedom of the system into a {\sl fast} and a {\sl slow} one, in
the classical limit, the rapid rotation of the particle around the guiding
center and the slow guiding center drift. In terms of these operators the
Hamiltonian of the system rewrites as a power series in the magnetic length
$\lb=\sqrt{\hbar c\over eB}$ and the fast and slow dynamics separates. The
effective guiding center Hamiltonian is obtained to the second order in the
adiabatic parameter $\lb$ and reproduces correctly the classical limit.
