A comprehensive account is given of the electrical double-layer interaction forces between spheres with the condition of either constant surface potential or constant surface charge density. The spheres may be of unequal size and the surface potentials at infinite separation of unequal magnitude and opposite sign. The Derjaguin approximation is used at small separations and the superposition formula for spheres at larger ones. In the Derjaguin approximation the double-layer force between two spheres is proportional to the interaction free energy of parallel plates and a discussion of the various constant charge cases for the latter is given. Simple formulas involving quadratures are derived for the plate interaction free energy at both constant potential and constant charge, and the difference formula of Frens is generalized. Transformations to standard elliptic integrals are given for the various constant charge situations. A number of curves of the force between spheres against separation are presented and the influence of geometry and of varying the constant potential or the constant charge density on one sphere is investigated. It is shown that the behavior of the electrical forces between two dissimilar surfaces at small separations can be complex, and many unusual effects may arise depending on which boundary condition is applicable. The small potential approximation is considered and found to be less effective at constant charge density than at constant potential.