  The van der Waals interaction between two ground-state atoms is calculated
for two electrically or magnetically polarizable particles embedded in a
dispersive magneto-dielectric medium. Unlike previous calculations which infer
the atom-atom interaction from the dilute-medium limit of the macroscopic,
many-body van der Waals interaction, the interaction is calculated directly for
the system of two atoms in a magneto-dielectric medium. Two approaches are
presented, the first based on the quantized electromagnetic field in a
dispersive medium without absorption and the second on Green functions that
allow for absorption. We show that the correct van der Waals interactions are
obtained regardless of whether absorption in the host medium is taken into
account.
