We present a general formalism for studying the cavity quantum electro
dynamics for an atom or a radiating dipole inside a hollow spherical c
avity. The atomic-energy-level shifts and the modified dipolar decay r
ates are calculated in specific cases. in particular, the energy shift
s for an atom located at the center of an empty sphere (radius a) vary
as 1/a(4) for large ka and 1/a(2) for small ka, where k is the wave n
umber. We attribute these variations to the retarded Casimir-Polder in
teraction and the state-independent vacuum-field contribution, respect
ively.