In the study of the derivation properties of interval functions, there are certain arguments that reappear in many settings. In this book, the author seeks to present a unified approach to some of these techniques. The motivation grows out of the interesting and important study of Rogers and Taylor characterizing those interval functions which are, in a sense, absolutely continuous with respect to the 8-dimensional Hausdorff measure. This problem leads naturally to an investigation of Lipschitz numbers Ds(f,x) = lim sup y,z ?x,y