Embedded networked sensors promise to revolutionize the way we interact with our physical envi- ronment and require scalable, ad hoc deployable and energy-ecient node localization/positioning. This paper describes the motivation, design, implementation and experimental evaluation (on sharply resource-constrained devices) of a self-con�guring localization system using radio beacons. We identify beacon density as an important parameter in determining localization quality, which saturates at a transition density. We develop algorithms to improve localization quality by (i) automating placement of new beacons at low densities (HEAP) and (ii) rotating functionality among redundant beacons while increasing system lifetime at high densities (STROBE).