Service oriented computing has gained a considerable momentum as a new paradigm for building enterprise information systems. Notable efforts have been made recently from both researchers and industrials to support the construction of service-based applications, nevertheless several issues still need to be tackled including service de?nition and adaptation, and services orchestration. This work proposes PYROS, an environment for building and orchestrating open services. An open service is represented by a workflow that coordinates calls to service provider methods. Thereby component activities and the way they are synchronized are rendered visible. In order to finely orchestrate services, they are associated with entry points. An entry point acts as a gateway for inserting and getting information about the progress of service execution. The paper details the approach adopted by PYROS for building and orchestrating services, and presents associated architectural choices. Furthermore, it reports an experimentation that we conducted for implementing an eTrader application using PYROS.