  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. 
Topics in quantum field theory: Renormalization groups in Hamiltonian   framework and baryon structure in a non-local QCD model