Dynamics of helium in restricted geometries

We review microscopic calculations for excitations, the dynamics of Andreev states, as well as neutron and atom scattering processes off helium mixtures, films, slabs, and clusters. Modern theoretical methods are built upon quantitative, microscopic descriptions of the ground state of both the host liquid as well as potential impurity atoms. A thorough analysis of the theoretical approach and the physical circumstances point towards the essential role played by multi-particle excitation and inelastic processes. In particular, inelastic processes determine almost exclusively atom scattering properties.