Dispersion forces : analysis of local field effects and inclusion of excitation In this thesis we have used macroscopic quantum electrodynamics to extend the concept of dispersion forces to objects in media and on excited systems.Our results facilitate a deeper understanding of dispersion interactions in the context of biological systems and colloid science.They stress the potential of amplification and left handed metamaterials for manipulating dispersion forces on atoms and bodies in nanotechnologies.We have first studied the asimir Polder potential of ground state atom in front of a planar interface between two magnetodielectric media where the local field correction is implemented via the Onsager real cavity model.In particular we have proposed an estimate of the on surface value of the potential.Secondly we have examined the Casimir Polder interaction of a ground state atom and a small magnetodielectric sphere in the presence of arbitrary magnetodielectric background media and bodies. We have proposed a model that is able to describe molecular systems of arbitrary size.To demonstrate the impact of negative refraction on the Casimir Polder potential and the spontaneous decay rate of an excited atom we have studied a superlens scenario.We have shown that an arbitrarily small but finite amount of material absorption drastically changes the Casimir Polder potential and the decay rate compared to the ideal scenario with vanishing absorption.We have further calculated the Casimir force on an amplifying but linearly responding magnetoelectric body. As we have shown amplification leads to resonant force components which can be exploited to create repulsive Casimir forces. Finally we have proven that the Casimir force on an optically dilute amplifying body can be calculated as a sum over the Casimir Polder forces on the excited atoms inside the body.