Baumgartner's Axiom postulates that any two $\aleph_1$-dense subsets of the real line are order-isomorphic. A set is $\aleph_1$-dense iff every nonempty open interval intersects the set in $\aleph_1$-many points. We present Todorcevic's argument which shows that Baumgartner's Axiom is a consequence of the Proper Forcing Axiom.