  After reviewing briefly the classical examples of duality in four dimensional
field theory we present a generalisation to arbitrary dimensions and to p-form
fields. Then we explain how U-duality may become part of a larger non abelian
V-symmetry in superstring/supergravity theories. And finally we discuss two new
results for 4d gravity theory with a cosmological constant: a new exact
gravitational instanton equation and a surprizing linearized classical duality
around de Sitter space.
