A monitoring device is configured to be attached to an agent to be monitored. The device includes sensor circuitry that detects or measures conditions relating to the environment, the agent or the agent's behavior. A microcontroller processes the conditions to produce event data, and a wireless transmitter transmits the event data to a remote host system. The conditions can include temperature, acceleration, the presence of another tag, or other conditions relating to the agent. The event data may relate to movement of the agent, an encounter with another tag, or other behavior of the agent, and may be stored to an internal memory. The monitoring device may also include a transmitter that transmits a signal, or tag identifier, that identifies the device. The event data may be reported as information relating to the behavior of the agent.