This thesis in computer science is situated more particulary in the field of knowledge engineering. It concerns the a posteriori setup of assistance systems in existing applications, while having a generic approach. In order to setup the setup of assistance systems in existing applications without a need to redevelop it or to access its source code, we choose to have a fully epiphytic approach. We proposed a adjunction process of an assistance system to a target-application with a epiphytic manner. It is constituted of two phases: the specification and the execution of the assistance. The assistance specification phase enables an expert, the assistance designer, to represent his knowledge relative to the target-application and to the assistance that he wishes to setup. The assistance execution phase uses this knowledge to provide the target-application end-users with the assistance wished by the designer. To make possible on the one hand the assistance specification by an assistance designer potentially non-computer scientist, and one the second hand the automatic execution of the specified assistance, we propose a pivot language: aLDEAS. This graphical language makes possible the definition of very varied assistance systems, with the shape of a set of rules. Our theoretical propositions have been implemented through the SEPIA system, constituted of different tools. The SEPIA assistance editor is aimed at assistance designers, and it implemented the assistance specification phase. It provided the assistance designers with an interface to handle aLDEAS elements in order to define assistance rules. These rules can then be executed by the SEPIA generic assistance engine, which implements the assistance execution phase. It provides the target-application end-users with the specified assistance. For this purpose, the assistance engine manages different epiphytic tools, in order to monitor and inspect the target-application, and to perform the assistance actions. The models implemented through the SEPIA system are generic, but it make possible the setup of assistance systems specifically suited on the one hand to their target-application, and on the second hand to the end-users.