This paper develops the perturbative aspects of affine Toda lagrangians. A renormalisation procedure suitable for these theories is discussed and then applied to some low order calculations, notably showing the on-shell vanishing of classically forbidden three particle couplings and verifying a coupling constant conjecture contained within suggested expressions for the exact S-matrices of these theories. Dispersion theory and elastic scattering are also examined. Throughout the calculations, new (often geometric) formulae satisfied by the highly constrained classical masses and couplings are discovered which are necessary for the many cancellations of the theory.