Physical systems are characterized by several energy scales. Effective
field theories explore a separation of scales in order to describe
low-energy phenomena, using the ratios of small-to-large scales as
controlled expansion parameters, even if no small coupling constants
exist. Particularly interesting are systems ---such as nucleons or 4He
atoms--- where two particles form a (real or virtual) bound state ---such
as the deuteron or the 4He dimer--- that is much larger than the range of
the interaction. In this case the three-body system presents remarkable
universal properties, such as various correlations among (three-particle)
binding energies and scattering observables. We discuss the surprising
properties of effective field theories applicable to this class of
systems, in particular the limit-cycle behavior of three-body forces.