  The Rubio de Francia extrapolation theorem is a very powerful result which
states that in order to show that certain operators satisfy weighted norm
inequalities with Muckenhoupt weights it suffices to see that the corresponding
inequalities hold for some fixed exponent, for instance $p=2$. In this paper we
extend this result and show that this extrapolation principle allows one to
obtain weighted estimates in tent spaces. From our extrapolation result we
automatically derive new estimates (and reprove some other) concerning
Calder\'on-Zygmund operators, operators associated with the Kato conjecture, or
fractional operators.
