Jewish law requires that in the production of dairy products, a mashgiach or Jewish supervisor must be present from the beginning of the milking to the end of processing to ensure that only milk from kosher animals is used. Where supervised milk is unavailable, some Rabbinic authorities permit government inspection as sufficient assurance (although not in all countries). All agree, however, that actual supervision is preferable. Milk with such supervision is known as chalav Yisrael.
Jewish tradition stresses the importance of using chalav Yisrael products exclusively, and emphasizes that using non-chalav Yisrael dairy products can have an adverse spiritual effect. Even when chalav Yisrael is very difficult to obtain, many people, aware of its positive effect on a Jew's spiritual sensitivity, go out of their way to acquire these products. Certainly, where they are readily available, one is required by Jewish law to use these products exclusively.