Wireless sensor networks are a promising technology for both sensing and controlling physical elements in the environment. Due to the limited freely available wireless spectrum, sensor networks have to compete with existing (generally more powerful) wireless devices. In this project we will investigate various approaches to help sensor networks co-exist with already deployed WiFi networks. Our goal is to develop a new MAC protocol that is able to both detect existing interference and coordinate packet transmissions in such a manner that collisions are reduced. We plan to demonstrate the effectiveness of this protocol on the FireFly wireless sensor networking platform.