  We discuss repulsive Casimir forces between dielectric materials with non
trivial magnetic susceptibility. It is shown that considerations based on naive
pair-wise summation of Van der Waals and Casimir Polder forces may not only
give an incorrect estimate of the magnitude of the total Casimir force, but
even the wrong sign of the force when materials with high dielectric and
magnetic response are involved. Indeed repulsive Casimir forces may be found in
a large range of parameters, and we suggest that the effect may be realized in
known materials. The phenomenon of repulsive Casimir forces may be of
importance both for experimental study and for nanomachinery applications.
