Buildings　accounted　for　half　of　global　electricity　consumption　in　recent　years.　Accurate　occupancy　information　could　improve　the　energy　efficiency　and　reduce　the　energy　consumption　in　built　environments.　Although　prior　studies　have　explored　various　sensing　techniques　for　occupancy　sensing,　these　solutions　still　suffer　from　serious　drawbacks,　e.g.　their　estimated　occupancy　information　is　coarse,　extra　infrastructure　is　required,　and　the　privacy　of　occupants　is　exposed.　In　this　paper,　we　present　the　design　and　implementation　of　a　novel　and　practical　occupancy　sensing　system,　WinOSS,　which　is　able　to　provide　fine-grained　occupancy　information　thoroughly　by　leveraging　existing　commodity　WiFi　infrastructure　along　with　the　WiFi-enabled　mobile　devices　carried　by　occupants.　We　have　implemented　WinOSS　in　a　1500　m(2)　built　environment　for　four　weeks　to　validate　its　performance.　Extensive　experimental　results　demonstrate　that　WinOSS　outperforms　existing　occupancy　sensing　techniques,　and　provides　comprehensive　fine-grained　occupancy　information　(including　occupancy　detection,　counting　and　tracking)　in　an　accurate,　reliable,　cost-effective　and　non-intrusive　manner.　(C)　2017　Elsevier　B.V.　All　rights　reserved.