Milk and meat of ruminants have a specific fatty acid composition due to (i) extensive biohydrogenation processes in the rumen, which is the main site of feed digestion, (ii) de novo synthesis and partial desaturation of absorbed fatty acids in the mammary gland, and (iii) partial desaturation and elongation during meat fat synthesis. These processes result in low concentrations of polyunsaturated fatty acids, including limited amounts of conjugated fatty acids, and in significant amounts of trans monounsaturated fatty acids. Ruminant dairy and meat products are the only foods which naturally contain conjugated fatty acids. The main trans isomer is vaccenic acid (t11-18:1) of which the effects on human health cannot be confounded with those of other industrial trans isomers from hydrogenated oils. It is possible to modulate fatty acid composition of milk and meat from ruminants through animal feeding.