  We present a study of several systems in which a large scale field is
generated over a turbulent background. These large scale fields usually break a
symmetry of the forcing by selecting a direction. Under certain conditions, the
large scale field displays reversals so that the symmetry of the forcing is
recovered statistically. We present examples of such dynamics in the context of
the dynamo instability, of two dimensional turbulent Kolmogorov flows and of
turbulent Rayleigh-B\'enard convection. In these systems reversals occur
respectively for the dynamo magnetic field, for the large scale circulation
generated by a periodic forcing in space and for the large scale roll generated
by turbulent thermal convection. We compare the mechanisms involved and show
that their properties depend on some symmetries of the system and on the way
they are broken.
