This paper describes the development of small mobile robots for collaborative surveillance tasks. Each of the robots, called Millibots, has only limited sensing, computation, and communication capabilities. However, by collaborating with other robots, they can still perform useful tasks. The task that we are considering is collaborative mapping and exploration inside buildings. To guarantee accessibility through narrow passageways, the robots are very small, approximately 6 by 6 by 6 cm. This size puts severe weight and power limitations on the design of the robots. To overcome these limitations, we are developing a modular system in which modules with different sensing, computation, and communication capabilities can be combined into a compete robot that is specifically designed for a given task. By making the design modular, we can avoid carrying around capabilities that are not essential for the current task. The concept of modularity also plays an important role in the design of the robot team. Here the 'modules' are the individual robots and the design task addresses the problem of determining how many robots to use and what kind of capabilities to select on different robots such that the overall team is capable of completing its task. The paper addresses these design issues and illustrates them with the specific example of the Millibot team.