  We discuss techniques of the density matrix renormalization group and their
application to interacting fermion systems in more than one dimension. We show
numerical results for equal--time spin--spin and singlet pair field correlation
functions, as well as the spin gap for the Hubbard model on two chains. The
system is a gapped spin liquid at half--filling and shows weak algebraic
$d$-wave--like pair field correlations away from half--filling.
