Milk components, with respect to the potential for improving health and the prevention of health problems, are intermediate products of the biohydrogenation of poly-unsaturated fatty acids in the rumen of cattle. Dairy products are a major source of conjugated linoleic acids (CLA) in the human diet. It has been demonstrated that CLA can suppress carcinogenesis, modulate the immune system, and can have an anti-obesity effect, while reducing atherogenesis and diabetes. Controlled intervention studies showed that trans fatty acids have a significant adverse effect on the LDL cholesterol level. The aim must therefore be to have a high content of poly-unsaturated fatty acids and CLA in milk, while concurrently having a low content of trans-9 and trans-10 C_18:1 fatty acids. The high correlation between CLA and the sum of all trans fatty acids of r = 0.88 in milk fat demonstrates that this goal is not easy to achieve. New feeding strategies for lactating ruminants in which an independent regulation of the different C_18.1 isomers is considered as pasture feeding, concentrate to roughage management, use of protected plant oils and use of milk fat low-sensitive cows have to be developed.