  The effective theory based on combined chiral and heavy quark symmetry
(HHChPT) is applied to D meson decays. The predictions for several flavor
changing neutral current rare radiative charm decays are given both in the
Standard Model as well as for the MSSM with and without R parity conservation.
Further, the quenched version of HHChPT is used to estimate effects of quenched
approximation in the lattice calculations of B-> pi, K transitions. In the
thesis several technical details are clarified as well. The explicit
calculation of three and four-point scalar functions with one heavy-quark
propagator is given. Also, existing renormalization group evolutions for B and
K meson decays are modified to perform next-to-leading order evolution of
Wilson coefficients for charm decays.
