The theory of Hopf algebras is closely connected with various applications, in particular to algebraic and formal groups. Although the rst occurence of Hopf algebras was in algebraic topology, they are now found in areas as remote as combinatorics and analysis. Their structure has been studied in great detail and many of their properties are well understood. We are interested in a systematic treatment of Hopf algebras with the techniques of forms and descent. The rst three paragraphs of this paper give a survey of the present state of the theory of forms of Hopf algebras and of Hopf Galois theory especially for separable extensions. It includes many illustrating examples some of which cannot be found in detail in the literature. The last two paragraphs are devoted to some new or partial results on the same eld. There we formulate some of the open questions which should be interesting objects for further study. We assume throughout most of the paper that k is a base eld and do not touch upon the recent beautiful results of Hopf Galois theory for rings of integers in algebraic number elds as developed in [C1].