The use of middleware is the current practice for developing distributed systems. Developers compose reusable services provided by standard middleware infrastructures, e.g., CORBA 1, DCOM2, Java RMI & re- lated services3, etc., to deal with problems like distribution, security, transactional processing, fault tolerance, etc.. The development process gets even easier after the evolution of the originally proposed Object-Oriented middleware paradigm, towards nowadays component-based middleware paradigm, e.g., CCM1, MTS2, EJB3. Developers do not have to burden with the, sometimes considerably complex, composition of different middleware services. Instead they build their middleware components and deploy them within of-the-shelf middleware containers realizing a customizable composition of middleware services. However, things are not as simple as they seem. Middleware vendors have to design and implement complex architectures combining available middleware services into a flexible/customizable way. Different ways of composing middleware services into a middleware architecture that satisfies application requirements are possible. The resulting compositions should be supported by the configurable middleware architecture that is provided by vendors to customers. Moreover, of-the-shelf middleware architectures should come along with a quality assessment of the different possible compositions they support. This shall give customers clues for selecting the most suitable middleware compositions for their particular systems. In this paper we address the above issues. More specifically, we present a developer-oriented environment that facilitates the design and quality analysis of flexible/configurable middleware architectures. The environment provides support for modelling middleware architectures. It further comprises a repository that is populated by a middleware vendor with basic middleware architectural patterns describing the use of individual services provided by the vendor's infrastructure. The environment further provides a tool which constructs all possible valid compositions of a set of basic middleware architectural patterns. Finally, the environment includes tool support for the automated generation of traditional quality models for the performance and reliability analysis of the different valid compositions of middleware architectural patterns. Those models serve as input to existing performance and reliability analysis tools, which are integrated into the environment.