  We explore the scattering approach to Casimir forces. Our main tool is the
description of Casimir energy in terms of transition operators, as presented in
Kenneth and Klich, Phys. Rev. Lett. 97, 160401 (2006). We study the convergence
properties of the formula and how to utilize it, together with scattering data
to compute the force. We illustrate the approach by describing the force
between scatterers in 1d and 3d,, and in particular show how it may be applied
in order to study the interaction between two spherical bodies at all
distances.
