A general three-dimensional boundary integral formulation for pile groups is used to obtain a set of dynamic interaction factors for a number of soil profiles and for a wide range of pile and soil parameters. The formulation employs the Green's functions for barrel loads and disk loads in layered semi-infinite media to couple the dynamic response of the soil mass to that of the piles by invoking the equilibrium and compatibility requirements. The interaction factors presented in this paper can be used in conjunction with the superposition method.....