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Polymers in confined environments show deviations from their bulk-properties, i.e. various size effects, due to the interference of intrinsic length scales with the dimensions of the imposed geometry. A solid understanding of the structural and dynamic properties of polymers in confined geometry becomes more and more a prerequisite for the successful use of polymers for nano-structured functional materials, cellular electrets and thin film processing for the microelectronics technology. This paper deals with effects of geometric confinement on the dynamics, particularly the glass transition dynamics of ultra-thin polymer films for which spectacular glass transition reductions up to 70 K have been reported, usually based on dilatometric techniques. Here we have employed dielectric relaxation spectroscopy (DRS) that was successfully applied to ultra-thin films (3 < L < 150 nm) of Polystyrene and Poly(methyl methacrylate). Due to its wide frequency range, DRS is able to provide information on various molecular relaxation processes in extremely confined polymer films and give insight on the molecular mobility at various length scales of the polymer chains. The DRS results revealed the importance of two main mechanisms that affect the Tg in ultra-thin polymer films: i) The "true" finite size effect that originates from the breakdown of cooperative motions for L < 5 nm resulting in the vanishing of the dynamic glass transition, and ii) chain confinement, i.e. conformational changes of entire polymer coils induced by "squeezing" the polymer chains below their characteristic length REE (end-to-end distance).