Middleware platforms have emerged as an effective answer to the requirements of open distributed processing. Existing middleware standards, however, do not fulfil important requirements of new application areas like multimedia and mobile computing, which require dynamic adaptability of the underlying platform. In our opinion, these requirements can be met by adopting an open engineering approach, allowing for inspection and adaptation of the platform's configuration. This paper presents an architecture for configurable and open middleware platforms based on computational reflection and multiple orthogonal meta-models. It also describes a first prototype, which implements the main points of the architecture and demonstrates its practicality.