  It is well known that the zeroth stable homotopy group of a genuine
equivariant commutative ring spectrum has multiplicative transfers (norms),
making it into a Tambara functor. We prove here that all Tambara functors can
be obtained in this way. In fact, we prove that the homotopy category of
Eilenberg MacLane commutative ring spectra is equivalent to the category of
Tambara functors. Several algebraic results on Tambara functors are derived as
corollaries. Finally, we rule out the existence of a lax symmetric monoidal
construction for Eilenberg MacLane spectra when the group is nontrivial.
