  We present the optimal estimation of an unknown squeezing transformation of
the radiation field. The optimal estimation is unbiased and is obtained by
properly considering the degeneracy of the squeezing operator. For coherent
input states, the r.m.s. of the estimation scales as $(2\sqrt {\bar n})^{-1}$
versus the average photon number $\bar n$, while it can be enhanced to $(2\bar
n)^{-1}$ by using displaced squeezed states.
