We study the behavior of D-branes at distances far shorter than the st
ring length scale l(s). We argue that short-distance phenomena are des
cribed 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 g(s) this corresponds to momenta and energies
far above string scale. We use 0-brane quantum mechanics to study 0-b
rane collisions and find structure at length scales corresponding to t
he eleven-dimensional Planck length (lp(11) similar to g(s)(1/3)l(s))
and to the radius of the eleventh dimension in M-theory (R(11) similar
to g(s)l(s)). We use 0-branes to probe non-trivial geometries and top
ologies at sub-stringy scales. We study the 0-brane 4-brane system, ca
lculating the 0-brane moduli space metric, and find the bound state at
threshold, which has characteristic size lp(11). We examine the blowu
p of an orbifold and are able to resolve the resulting S-2 down to siz
e lp(11). A 0-brane with momentum approaching 1/R(11) is able to explo
re a larger configuration space in which the blowup is embedded. Analo
gous phenomena occur for small instantons. We finally turn to 1-branes
and calculate the size of a bound state to be similar to g(s)(1/2)l(s
), the 1-brane tension scale.