  In this review we give a perspective of the theoretical work done recently on
the interpretation of results from $B$, $D$, $\Lambda_b$, $\Lambda_c$ weak
decays into final states that contain interacting hadrons, and how it is
possible to obtain additional valuable information that is increasing our
understanding of hadron interactions and the nature of many hadronic
resonances. The novelty of these processes is that one begins with a clean
picture at the quark level which allows one to select the basic mechanisms by
means of which the process proceeds. Finally, one has a final state described
in terms of quarks. To make contact with the experiments, where mesons and
baryons are observed, one must hadronize, creating pairs of $q \bar q$ and
writing the new states in terms of mesons and baryons. This concludes the
primary hadron production in these processes. After that, the interaction of
these hadrons takes place, offering a rich spectrum of resonances and special
features from where it is possible to learn much about the interaction of these
hadrons and the nature of many resonances in terms of the components of their
wave functions.
