This report introduces a new sensor network simulator for the Contiki OS - the COOJA Simulator. The Contiki OS is a portable operating system designed specifically for resource limited devices such as sensor nodes. It is built around an event-driven kernel but supports pre-emptive multithreading at a per-process basis. It also supports a full TCP/IP stack via uIP and the programming abstraction Protothreads. The main design goal of the COOJA Simulator is extendibility, for which Interfaces and Plugins are used. An Interface represents a sensor node property or device, such as a position, a button or a radio transmitter. A Plugin is used to interact with a simulation, for example to control the simulation speed or to watch all network traffic between the simulated nodes. Both new Plugins and Interfaces can easily be created and added to the simulation environment. A number of other parts of the simulator, for example a radio medium responsible for forwarding radio network data, can also be implemented and added to the simulator. And by supporting several different simulation environments at the same time in one simulation, different underlying hardware platforms can be simulated in heterogeneous networks. Java Native Interface is used to connect the new simulator with Contiki, allowing simulated applications to run in a real Contiki system. By using this approach, any simulated application can then be run on a real sensor node unaltered.