Multi-agent based simulations (MABS) of real world scenarios are attracting growing interest. Complex real world scenarios require deep knowledge and expertise which can only be provided by specialists in the application area. However, it cannot be expected that such experts understand agent-based technology and simulation. Consequently, tools are required, which deliver a high level, easy usable interface. In this article we propose a new simulation framework based on the JADE framework. Besides extensions to deal with the time aspect, agent/environment interaction, visualization and load balancing, we also address the usability of the tool for specialists from different domains. For this, our framework, called Agent. GUI, provides an easy to use, customizable graphical user interface. Overall, Agent. GUI is a powerful tool for the development of multi-agent based simulations.