Specialization is a technique that has the potential to provide operating system clients with the performance and functionality that they need, while still retaining the advantages of a simple generic code base for the operating system maintainer. However, at present the specialization process is labor-intensive and requires the knowledge of an expert in the domain of application behavior. In order to realize the full advantages of specialization, we believe that the process must be automated. This means building tools for specialization, and also making the domain knowledge explicit in some form or other. A specialization toolkit has been developed jointly at the Oregon Graduate Institute and IRISA, as part of the Synthetix project. This paper discusses our preliminary ideas on the use of Microlanguages to describe application behavior and to make that information available to the specialization tools. 1