  Traditional estimators of the mass of galaxy clusters assume that the cluster
components (galaxies, intracluster medium, and dark matter) are in dynamical
equilibrium. Two additional estimators, that do not require this assumption,
were proposed in the 1990s: gravitational lensing and the caustic technique.
With these methods, we can measure the cluster mass within radii much larger
than the virial radius. In the caustic technique, the mass measurement is only
based on the celestial coordinates and redshifts of the galaxies in the cluster
field of view; therefore, unlike lensing, it can be, in principle, applied to
clusters at any redshift. Here, we review the origin, the basics and the
performance of the caustic method.
