This paper presents the MIMO MIddleware MOnitoring system, an infrastructure for monitoring and managing distributed, heterogeneous middleware environments. MIMO is based on a new multi-layer-monitoring approach for middleware systems, which classifies collected information using several abstraction levels. The key features of MIMO are its openness, flexibility, and extensibility. MIMO’s research contribution is to enable easy integration of heterogeneous middleware platforms, to be suited for large classes of online tools covering both monitoring and management functionality, and therefore to be applicable for tools supporting the complete software lifecycle. In addition to the core MIMO system we outline exemplary instrumentation techniques for integrating CORBA and DCOM platforms, and present the MIVIS visualization tool demonstrating the features of the MIMO infrastructure.