This paper describes a general approach for the integration of arbitrary declarative languages and constraint systems. The main idea is to consider declarative programs together with the language evaluation mechanisms as constraint solvers and to integrate them into an overall system of cooperating solvers. Exemplarily, we present the integration of a logic language with a constraint system, and the extension of a functional logic language with constraints. The approach allows to build constraint languages according to current requirements and thus it enables comfortable modelling and solving of many problems.