  The quasinormal modes (QNM's) of gravitational systems modeled by the
Klein-Gordon equation with effective potentials are studied in analogy to the
QNM's of optical cavities. Conditions are given for the QNM's to form a
complete set, i.e., for the Green's function to be expressible as a sum over
QNM's, answering a conjecture by Price and Husain [Phys. Rev. Lett. {\bf 68},
1973 (1992)]. In the cases where the QNM sum is divergent, procedures for
regularization are given. The crucial condition for completeness is the
existence of spatial discontinuities in the system, e.g., the discontinuity at
the stellar surface in the model of Price and Husain.
