  Using modern nucleon-nucleon interactions in the description of the $A=3,4$
nuclei, it is not possible to reproduce both the three- and four-nucleon
binding energies simultaneously. This is one manifestation of the necessity of
including a three-nucleon force in the nuclear Hamiltonian. Several models of
the three-nucleon force exist and are applied in the description of light
nuclei. However, as it is discussed here, a simultaneous description of the
three- and four-body binding energies and the $n-d$ doublet scattering length
seems to be problematic. Accordingly, a comparative study of some of these
models is performed. In a different analysis, we study applications of the Kohn
Variational Principle, formulated in terms of integral relations, to describe
$N-d$ scattering processes.
