  We show that unstable D-branes play the role of ``D-sphalerons'' in string
theory. Their existence implies that the configuration space of Type II string
theory has a complicated homotopy structure, similar to that of an infinite
Grassmannian. In particular, the configuration space of Type IIA (IIB) string
theory on $\R^{10}$ has non-trivial homotopy groups $\pi_k$ for all $k$ even
(odd).
