In this paper, we present CASi (Context Awareness Simulator), a software system for the simulation of context-aware computer systems and environments. CASi provides an abstract framework of components for simulating smart world applications like a smart office or house with ambient sensors and actuators. Agents moving through these application worlds are tracked by sensors and their actions are influenced by actuators, both of which can be programmed to resemble the actual peripherals of the tested system. CASi allows testing ambient, context aware computer systems even in early stages of development without the need for expensive prototyping or real world deployment.