  We develop a theory which describes hybrid structures consisting out of
superconducting and ferromagnetic parts. We give two examples for applications.
First, we consider a hybrid structure containing a strong ferromagnet in the
ballistic limit. Second, we study for a weak ferromagnet the influence of a
domain wall on the superconducting proximity effect. In both cases we account
quantitatively for the mixing between singlet and triplet correlations.
