We study the behavior of D-branes at distances far shorter than the string length scale
ls. We argue that short-distance phenomena are described by the IR behavior of the D-brane world-volume quantum theory. This description is valid until the brane motion becomes relativistic. At weak string coupling
gs this corresponds to momenta and energies far above string scale. We use O-brane quantum mechanics to study O-brane collisions and find structure at length scales corresponding to the eleven-dimensional Planck length (
l11p gs ls) and the radius of the eleventh dimension in M-theory (
R11 gsls). We use O-branes to probe non-trivial geometries and topologies at sub-stringy scales. We study the O-brane 4-brane system, calculating the O-brane moduli space metric, and find the bound state at threshold, which has characteristic size
l11p. We examine the blowup of an orbifold and are able to resolve the resulting
S2 down to size
l11p . A O-brane with momentum approaching 1/
R11 is able to explore a larger configuration space in which the blowup is embedded. Analogous phenomena occur for small instantons. We finally turn to 1-branes and calculate the size of a bound state to be
gsls, the 1-brane tension scale.