There appears to be some confusion as to how margarine is defined. Margarine is a butter substitute made with vegetable oils. Both butter and margarine contain 80% fat, so margarine can generally be used in place of butter with acceptable results. Our home economists use butter, not margarine, when testing recipes for a variety of reasons. In some recipes, we feel that butter adds a rich flavor that margarine would not achieve, like in a butter cookies recipe, for example.