Abstract. We give a computational description of Hensel's method for lifting approximate factorizations of polynomials. The general setting of valuation rings provides the framework for this and the other results of the paper. We describe a Newton method for solving algebraic and differential equations. Finally, we discuss a fast algorithm for factoring polynomials via computing short vectors in modules.1. Introduction. Hensel and Newton methods have received quite a lot of attention in algebraic computing. We present them in their natural framework, that of valuation rings. The Hensel method deals with factorization of polynomials, the Newton method with zeros of polynomials over the given valuation ring. Both methods take an approximate solution and produce a new approximation which is better with respect to the given valuation. Apart from the pioneering paper by Zassenhaus [1969], these methods have usually only been treated in the setting of either the integers or a polynomial ring, thus requiring separate proofs for each case. The unified treatment avoids this, and incidentally obtains the Newton method as a special case of the Hensel method, also giving the aesthetical advantage of avoiding rational functions for the important application of inverting power series.The Hensel method presented in Section 2 describes a lifting of an approximate factorization of a given polynomial over a valuation ring, where the factors are approximately relatively prime. It results in two choices of an iterative procedure, one with linear and one with quadratic convergence behavior. It allows us to describe the factorization of certain polynomials that are not squarefree over the residue class field, a case not covered by the usual formulation.In Section 3, we present a Newton method for solving differential equations for formal power series in several variables, in the general case of systems of nonlinear partial differential equations. This includes the case of a system of algebraic equations. One obtains a simple condition which provides an iterative procedure to compute a solution.In Section 4, we discuss an important recently discovered tool for factoring polynomials: computing short vectors in modules over (valuation) rings. This tool has been introduced by Lenstra-Lenstra-Lovasz [1982] for factoring univariate integer polynomials, used in Chistov-Grigoryev [1982], for multivariate polynomials over finite fields, and in Lenstra [1983a] for multivariate integer