i will discuss the problem of backwards-uniqueness or "unique-continuation" for the ricci flow, and prove that two complete solutions $g(t)$, $\tilde{g}(t)$ to the ricci flow on $[0, t]$ of uniformly bounded curvature that agree at $t=t$ must agree identically on $[0, t]$. a consequence is that the isometry group of a solution to the ricci flow cannot expand in finite time.