We explain our proposed wireless security and monitoring system based on space-time signal processing. The proposed system monitors a whole room with only a pair of transmitter and receiver. Using an antenna array at the receiver, we can get not only received signal strength (RSS) that is susceptible to noise but also eigenvector spanning signal subspace that is inherent to its environment. When an event (e.g. intrusion) occurs, the propagation environment changes, and thus the eigenvector changes. Based on the change of eigenvector, we detect an event. We explain our proposed system and show some experimental results.