An object-oriented methodology for the design of reusable model libraries, dedicated to the composition of hybrid simulation models, is proposed. It takes into consideration all the phases of the library life cycle: design, programming, maintenance, modifications, extension, etc. The methodology distinguishes between the role of the library designers and the role of the library users. One of its main goals is guaranteeing that the users are able to use the libraries without having to know their internal details. In particular, users should not be confronted with numerical problems. The library designers should guarantee the numerical efficiency of the models based on the library predefined-models. In this respect, some of the numerical aspects of the model library design are discussed. The concept of library design rules is introduced as the cornerstone of the proposed methodology.