In decay processes of B mesons the so-called "heavy light currents" play an
important role, for example in the determination of matrix elements of the CKM
mixing matrix.
The present thesis investigates various aspects of the strong interaction of the
quarks participating in the heavy light current. The theory of strong
interaction is Quantum Chromodynamics, which is treated in perturbation theory.
After a short introduction to the relevant effective field theories, which are
derived from QCD, some new results are presented. First the asymptotic behaviour
of the perturbative series is investigated, and the second result are proofs of
factorization theorems which are used to separate the perturbative from the nonperturbative
contributions.
Finally the symmetries of the effective field theories are used to find
relations between non-perturbative matrix elements involved in "heavy to light"
currents.