This book introduces multi-robot strategies making multiple robots explore an unknown environment in a cooperative manner. Multiple robots build a topological map of the environment, while deploying sensor nodes which can sense and communicate. As the sensor network built by one robot meets the network built by another robot, both robots can exchange data with each other, which leads to time-efficient exploration. Our networking strategy results in the sensor network without coverage holes and does not require the global localization of a node or a robot. Once the sensor network is built in an environment, we can utilize the network as the basis to protect the environment. We introduce an intruder capture algorithm assuming that a robot is able to access any intruder's location utilizing the sensor network. This algorithm is robust to time delay in information sharing based on the sensor network. Utilizing the intruder capture algorithm, we derive upper bounds on the number of robots needed to capture every intruder in the environment.