  Higher dimensional generalisations of self-duality conditions and of theta
angle terms are analysed in Yang-Mills theories. For the theory on a torus, the
torus metric and various antisymmetric tensors are viewed as coupling constants
related by U-duality, arising from background expectation values of
supergravity fields for D-brane or matrix theories. At certain special points
in the moduli space of coupling constants certain branes or instantons are
found to dominate the functional integral. The possibility of lifting chiral or
supersymmetric theories to higher dimensions is discussed.
