  A rigourous theory for the determination of the van der Waals interactions in
colloidal systems is presented. The method is based on fluctuational
electrodynamics and a multiple-scattering method which provides the
electromagnetic Green's tensor. In particular, expressions for the Green's
tensor are presented for arbitrary, finite, collections of colloidal particles,
for infinitely periodic or defected crystals as well as for finite slabs of
crystals. The presented formalism allows for {\it ab initio} calculations of
the vdW interactions is colloidal systems since it takes fully into account
retardation, many-body, multipolar and near-fields effects.
