Research results in wireless sensor networks are primarily gained from simulations and theoretical considerations. Currently, the community begins to realize that the results need to be validated in testbeds. Testbeds can also be used to directly gather knowledge with sensor network experiments on real hardware and a real environmental context. This survey gives an overview of different approaches to build testbeds and experimentation environments regarding different research foci. We discuss emerging testbed requirements and present existing solutions of the community. The overview is complemented with a discussion of common design decisions concerning architectures and experimentation support in current testbeds. A look on future trends and developments in wireless sensor network testbeds concludes this work. This survey is intended to help researchers to attain own results in real world experiments with wireless sensor networks. The reader gains an comprehensive overview on existing testbeds and practical knowledge documented in the referenced literature. The survey is laying the foundation for design decisions while developing an own testbed using the examples of described approaches.