This document aims to highlight the issues affecting the co-existence of IEEE 802.15.4-based systems in the presence of interference. The measures employed by the 802.15.4 standard to ensure reliable co-existence are outlined. The practical performance of an IEEE 802.15.4-based system is established with reference to supporting empirical and simulated data. Finally, guidelines are provided for installing sensor networks in either a planned or an unplanned RF environment.