Let us consider a small domain H in the complex plane, z e/7 and an immersion F:/7-.-,R 3 of/-/into Euclidean space. It is well known that if F is a conformal parametrization of a surface with constant mean curvature (CMC) and/7 is a neighborhood of a nonumbilic point then the metric g = 4eUdzd~ induced on H via F satisfies the elliptic sinh-Gordon equation uz~ + sinhu = 0.(1.1)Conversely, every solution of the Eq. (1.1) gives rise to CMC immersion. It is very important that here the elliptic sinh-Gordon equation may be applied not only in differential geometry in the small but also in differential geometry in the large. In particular, special doubly periodic solutions of this equation describe all CMC tori.Wente was the first [48] to show the existence of compact surfaces of genus one in R 3 with CMC, he constructed a countable number of isometrically distinct examples using a special solution of (1.1). These examples solved the long standing problem of Hopf [31]: Is a compact CMC surface in R 3 necessarily a round sphere?Important previous results were due to Alexandrov [3] and Hopf [31]. Alexandrov showed that if a CMC surface is embedded then it must be a round sphere. Hopf showed that an immersion of S 2 into R a with CMC, must be a round sphere.Wente's paper was followed by the series [1,43,45,49] where Wente tori were investigated in detail and other examples of CMC toil were constructed. All these modern results were obtained with the help of analytical investigation of special solutions of (1.1). In particular, Abresch described [1] all CMC tori having one family of planar curvature lines in terms of elliptic integrals. Walter [45] obtained more explicit representation of these tori in terms of elliptic and theta-functions of Jacobi type.