Demand for milk has waxed and waned over the last 100 years in response to changing perceptions of its health effects. Milk consumption was promoted for health benefits in the first half of the twentieth century, whilst milk fat has increasingly been regarded as something to avoid over the last 30 years. Emerging research is showing that milk fat provides a number of important components, almost uniquely, within a balanced human diet. Understanding of the role of animal diets in controlling milk fat content and milk fatty acid profiles has grown over this period. The multiple correlated changes associated with milk fat depression have led to a number of mechanistic theories which have not been resolved completely. The detailed mechanisms at the molecular level remain to be elucidated. Interestingly, the two research areas of milk fat content and milk fatty acid profiles have merged as it became clear that some of the intermediates of rumen biohydrogenation are involved in regulating milk fat content. The multivariate nature of milk fatty acid profiles means that future studies must make use of multivariate statistical techniques. These approaches will also be of great value in assessing the consequences of fatty acids for human health, where studies of the effects of single nutrients can be misleading. Issues about the sustainability of the marine harvests mean that attention needs to focus on alternative sources to meet the growing demand for n-3 fatty acids, notably from forages. Whilst attention has focused on milk fatty acids for their effects on human health, future work should also address effects on health and reproductive function of cows offered diets designed to alter milk fatty acid profiles.