  Complex molecules are intriguing objects at the interface between quantum and
classical phenomena. Compared to the electrons, neutrons, or atoms studied in
earlier matter-wave experiments, they feature a much more complicated internal
structure, but can still behave as quantum objects in their center-of-mass
motion. Molecules may involve a large number of vibrational modes and highly
excited rotational states, they can emit thermal photons, electrons, or even
atoms, and they exhibit large cross sections for collisional interactions with
residual background gases. This makes them ideal candidates for decoherence
experiments which we review in this contribution.
