  In this talk I briefly review the theory of resonances dynamically generated
from hadron-hadron scattering, sometimes referred to as "molecules"S. I give
some classical examples of meson-meson and meson-baryon systems, as well as a
few examples of different approaches to describe the interaction between
hadrons. To conclude, I comment on a few recent works that suggest that some of
the five new narrow $\Omega_c$ states, recently discovered by the LHCb
collaboration, can be interpreted as meson-baryon molecules.
