  We discuss the field theory of 3-brane probes in F-theory compactifications
in two configurations, generalizing the work of Sen and of Banks, Douglas and
Seiberg. One configuration involves several parallel 3-brane probes in F-theory
compactified on $T^4/Z_2$, while the other involves a compactification of
F-theory on $T^6/Z_2 x Z_2$ (which includes intersecting $D_4$ singularities).
In both cases string theory provides simple pictures of the spacetime theory,
whose implications for the three-brane world-volume theories are discussed. In
the second case the field theory on the probe is an unusual N=1 superconformal
theory, with exact electric-magnetic duality. Several open questions remain
concerning the description of this theory.
