The development of modeling tools within the context of future open computer-aided process engineering environments requires careful consideration of tool integration strategies such that the modeling process is optimally supported, and the resulting models achieve their full usage potential. We differentiate between five kinds of tool integration, describe the kinds of integration offered by existing process engineering environments, and thereby identify the major shortcomings of current approaches. To indicate a possible solution to these shortcomings we sketch the tool integration approach realized in PRO-ART/CE, a prototypical process engineering environment.