  I explore the possibility of using heavy meson decays as probes for flavour
changing neutral transitions (FCNC) $c\to u\gamma$ and $c\to ul^+l^-$. In the
standard model, these are the most frequent FCN transitions among the quarks
with charge $2/3e_0$. I predict the short and long distance contributions to
the relevant heavy meson decays within the standard model and explore their
sensitivity to different scenarios of physics beyond the standard model. The
rare $B_c\to B_u^*\gamma$ decay is proposed as the most suitable case study of
the $c\to u\gamma$ transition and is studied with the ISGW model. Its detection
at the branching ratio much higher than $10^{-8}$ would signal new physics.
Weak decays of charm mesons to a light meson and a photon or a charged lepton
pair are also studied for this purpose. Their probabilities are found to be
dominated by the long distance contributions and raise our hopes that they may
be detected soon. A window of opportunity to probe the $c\to ul^+l^-$
transition is found in the decay $D\to \pi l^+l^-$, at the kinematical region
of high di-lepton mass. In order to study the charm meson decays, I adapt the
Heavy Meson Chiral Lagrangian approach and demonstrate for its applicability by
predicting the charm meson nonleptonic decay rates.
