Testbeds as a means to evaluate protocol and software development are gaining importance, not least because of the oftentimes unpredictable influence of environmental behaviour. IBBT, the Interdisciplinary Institute for Broadband Technology, recognizes the importance of such testbeds and has therefore invested in WiLab.t, a wireless sensor and mesh testbed. It contains over 200 wireless and programmable nodes. The monitoring and management of such a testbed is very important so as to guarantee a proper functioning and stable environment to be used by researchers. This is however not a trivial task, even more so when in the future, the testbed is expanded with new devices and as such becomes a heterogeneous environment. Therefore, we have developed an ontology-based monitoring approach, which allows hiding the heterogeneity from the monitoring application and enables to process the data in a formal manner. Additionally, it allows adaptation according to characteristics of the local deployment, without the need to re-engineer the entire monitoring application every time alterations are made to the testbed.