  The numerical studies of d_{x^2-y^2}-wave pairing in the two-dimensional (2D)
and the 2-leg Hubbard models are reviewed. For this purpose, the results
obtained from the determinantal Quantum Monte Carlo and the density-matrix
renormalization-group calculations are presented. These are calculations which
were motivated by the discovery of the high-T_c cuprates. In this review, the
emphasis is placed on the microscopic many-body processes which are responsible
for the d_{x^2-y^2}-wave pairing correlations observed in the 2D and the 2-leg
Hubbard models. In order to gain insight into these processes, the results on
the effective pairing interaction as well as the magnetic, density and the
single-particle excitations will be reviewed. In addition, comparisons will be
made with the other numerical approaches to the Hubbard model and the numerical
results on the t-J model. The results reviewed here indicate that an effective
pairing interaction which is repulsive at (pi,pi) momentum transfer and
enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of
the Brillouin zone create optimum conditions for d_{x^2-y^2}-wave pairing.
These are two effects which act to enhance the d_{x^2-y^2}-wave pairing
correlations in the Hubbard model. Finding additional ways is an active
research problem.
