The zeta function continuation method is applied to compute the
Casimir energy on spheres SN. Both odd and even dimensional spheres are studied.
For the appropriate conformally modified Laplacian A the Casimir energy isshowntobefiniteforalldimensionseventhough,generically,itshoulddivergeinodddimensionsduetothepresenceofapoleintheassociatedzetafunctionζA(s).Theresidueofthispoleiscomputedandshowntovanishinourcase.AnexplicitanalyticcontinuationofζA(s)isconstructedforallvaluesofN.Thisenablesustocalculate exactly and we find that the Casimir energy vanishes in all even
dimensions. For odd dimensions δ is never zero but alternates in sign as N increases
through odd values. Some results are also derived for the Casimir energy of
other operators of Laplacian type