  Talk presented in the 97 Karpatcz winter school. We describe Sen's work on F
theory on K3 and its reflection on the world-volume field theory on a D3-brane
probe. Field theories on a multiple of probes are analyzed. We construct a 4d
N=1 superconformal probe theory which is invariant under electric-magnetic
duality via a compactification to six dimensions on T^6/Z_2 x Z_2.
