The design and analysis of multi-agent systems is difficult due to complex agent capabilities and rich interactions among agents. Experimentation is a crucial step in gaining insight into the behavior of agents. Experiments must be flexible, easily configurable, extensible, and repeatable. This paper presents the Sensible Agent Testbed, which supports these requirements. The CORBA infrastructure of the Testbed platform and the formally-defined interfaces among Testbed components are described. The Testbed promotes many levels of modularity, facilitating parallel development of agents and agent capabilities. This approach provides many opportunities for different types of experiments. Experimental setup through a configuration file is simplified using the Init File Maker, which has the capability to automate the production of multiple configurations. Overall, the Sensible Agent Testbed provides a solid infrastructure supporting multi-agent experiments.