  The essential decorrelation rate of a hyperbolic dynamical system is the
decay rate of time-correlations one expects to see stably for typical
observables once resonances are projected out. We define and illustrate these
notions and study the conjecture that for observables in $C^1$, the essential
decorrelation rate is never faster than what is dictated by the {\em smallest}
unstable Liapunov multiplier.
