  We present a simple formalism for the evaluation of the Casimir energy for
two spheres and a sphere and a plane, in case of a scalar fluctuating field,
valid at any separations. We compare the exact results with various
approximation schemes and establish when such schemes become useful. The
formalism can be easily extended to any number of spheres and/or planes in
three or arbitrary dimensions, with a variety of boundary conditions or
non-overlapping potentials/non-ideal reflectors.
