The role of milk in nature is to nourish and provide immunological protection for the mammalian young. Milk has been a food source for humans since prehistoric times; from human, goat, buffalo, sheep, yak, to the focus of this section - domesticated cow milk (genus Bos). Milk and honey are the only articles of diet whose sole function in nature is food. It is not surprising, therefore, that the nutritional value of milk is high. Milk is also a very complex food with over 100,000 different molecular species found. There are many factors that can affect milk composition such as breed variations, cow to cow variations, herd to herd variations - including management and feed considerations, seasonal variations, and geographic variations. To those with a good knowledge of chemistry and physics the various formulae and equations will prove useful, but it has been the endeavour of the author to treat the subject-matter in such a manner as to enable the salient facts to be appreciated by those with only an elementary knowledge of these two subjects.