The declining consumption of ruminant products has been partly associated with their high proportion (but not necessarily content) of saturated fatty acids. Recent studies have focused on the less prominent fact that they are also important sources of beneficial fatty acids, including n-3 fatty acids and conjugated linoleic acids. alpha-Linolenic acid (18 : 3n-3) is of particular interest because it also contributes to improved flavour of beef and lamb. Many recent studies showed large effects of special concentrates on levels of fatty acids in milk and meat. However, the 'rumen protection' treatments, needed to ensure a worthwhile level of fatty acid in products, are expensive. Herbage lipids are the cheapest and safest source of these fatty acids and so breeding to increase delivery of fatty acids from plants into ruminant products is an important long-term strategy. Plant lipids usually contain high levels of polyunsaturated fatty acids, particularly 18 : 2n-6 and 18 : 3n-3 which are the precursors of beneficial fatty acids. Whilst some plants are particularly rich in individual fatty acids (e.g. 18 : 3n-3 in linseed), there are also useful levels in grass and clover (Trifolium Spp.). Levels of fatty acids in forages in relation to species and varieties are considered, as well as management and conservation methods. Relationships between levels of fatty acids and existing traits and genetic markers are identified. The effects of forage treatments on the fatty acid content of ruminant products are reviewed. The higher levels of polyunsaturated fatty acids in milk from cows fed clover silages show that the level of fatty acids in herbage is not the only factor affecting levels of fatty acids in ruminant products. Further effort is needed to characterise susceptibility of unsaturated fatty acids to oxidative loss during field wilting and biohydrogenation losses in the rumen, and the relative importance of plant and microbial processes in these losses. The pathways of lipolysis and lipid oxidation are reviewed and other plant factors which offer potential to breed for reduced losses are considered.