Middleware platforms have emerged as an effective answer to the requirements of open distributed processing. However, in our opinion, a new engineering approach based on configurability and openess of platform implementations is essential to meet the needs of applications areas such as multimedia, groupware and mobile computing. This paper outlines our architecture for configurable and open middleware platforms, along with a first prototype. The architecture is based on the concept of reflection, the ability for a program to access, reason about and alter its own implementation in a principled way, according to a well-defined Meta-Object Protocol (MOP) [1].