  Gravitational lensing of distant objects caused by gravitational tidal forces
from inhomogeneities in the universe is weak in most cases, but it is noticed
that it gives a great deal of information about the universe, especially
regarding the distribution of dark matter. The statistical values of optical
quantities such as convergence, amplification and shear have been derived by
many people using various approaches, which include the linear perturbational
treatment in the weak limit and the nonlinear treatment considering small-scale
matter distribution.
  In this review paper we compare the following three main approaches: (a) the
approach in the multi-lens-plane theory; (b) the approach due to the direct
integration method; and (c) the perturbational approach.
  In the former two approaches inhomogeneous matter distributions are produced
in the CDM model using $N$-body simulations (the P$^3$M code and the tree-code,
respectively). In (c) the power spectrum corresponding to the CDM model is used
for the large-scale matter distribution.
