  We introduce a new theoretical approach to dissipative quantum systems. By
means of a continuous sequence of infinitesimal unitary transformations, we
decouple the small quantum system that one is interested in from its
thermodynamically large environment. This yields a trivial final transformed
Hamiltonian. Dissipation enters through the observation that generically
observables ``decay'' completely under these unitary transformations, i.e. are
completely transformed into other terms. As a nontrivial example the spin-boson
model is discussed in some detail. For the super-Ohmic bath we obtain a very
satisfactory description of short, intermediate and long time scales at small
temperatures. This can be tested from the generalized Shiba-relation that is
fulfilled within numerical errors.
