In this paper, we describe the opportunistic localization, which enables localization services that works seamlessly in heterogeneous environments including indoors as oppose to GPS based outdoor-only systems. Categories and Subject Descriptors C.5.3 [Computer System Implementation]: Microcomputers – Portable devices (e.g., laptops, personal digital assistants), Personal computers General Terms Algorithms, Design, Reliability, Experimentation Keywords opportunistic localization, smart phone, localization, tracking, data fusion, particle filter