—Observing the environment is the raison d’ ˆetre of sensor networks, but the precise reconstruction of the measured process requires too many messages for a low power sensor network. The number of messages can be reduced by focussing on events relevant for the application. This paper examines three event definitions for a home automation example and compares the message reduction. In the extreme case, the number of transmitted messages drops from more than 3000 messages per node and hour to a maximum of about two messages per node and hour.