  In this thesis we investigate aspects of two problems. In the first part of
this thesis, we concentrate on renormalization group methods in Hamiltonian
framework. We show that the well-known coupled-cluster many-body theory
techniques can be incorporated in the Wilsonian renormalization group to
provide a very powerful framework for construction of effective Hamiltonian
field theories. Eventhough the formulation is intrinsically non-perturbative,
we have shown that a loop-expansion can be implemented. As illustrative
examples, we apply our formulation on the $\Phi^{4}$ theory and an extended Lee
model. The many-body problem in an extended Lee model is also studied. We show
that a combination of the coupled-cluster theory and the Feshbach projection
techniques leads to a renormalized generalized Brueckner theory. The second
part of the thesis is rather phenomenologically orientated. In this part, we
will employ an effective field-theoretical model as can be constructed by means
of the techniques of the first part of the thesis, a quark-confining non-local
NJL model and study the baryon and diquarks in this model. After truncation of
the two-body channels to the scalar and axial-vector diquarks, a relativistic
Faddeev equation for nucleon bound states is solved in the covariant
diquark-quark picture. We study the possible implications of quark confinement
for the description of the diquarks and the nucleon. We also examine
alternative field theoretical approaches for describing baryons.
