%We study $o$-minimal expansions of the real field and apply it to %its Pfaffian closure. We remove in doing so the usual $\Cinf$ %assumption, exactly to the extend to which this is possible. % Using a modification of Wilkie's recent proof of o-minimality for Pfaffian functions, we give an invariant characterization of o-minimal expansions of $\IR$. We apply this to construct the Pfaffian closure of an arbitrary o-minimal expansion of $\IR$.