The linearized Poisson-Boltzmann theory is used to calculate the electrical double layer interaction free energy, as well as the double layer force, between unequal spherical colloidal particles. Results are given for interaction under conditions of constant surface potential, for constant surface charge, and for the case in which charge regulation due to the dissociation of surface groups may be modeled by a linear relationship between the surface charge and the surface potential. The interaction energy and force between a spherical colloidal particle and a plate, which has particular relevance to force measurements with the atomic force microscope and particle deposition studies, are also calculated. The results are used to test the accuracy of the linear Deryaguin approximation which forms the basis of the Hogg-Healy-Fuerstenau formula. The validity of the linearized Poisson-Boltzmann theory is tested against a numerical solution of the nonlinear Poisson-Boltzmann equation in the special case bf spherical particles of the same size but opposite surface potentials.