Passive, Privacy-Preserving Real-Time Counting of Unmodified Smartphones via ZigBee Interference The continuing proliferation of smartphones makes them an effective means to monitor the number of people within an area, for example, to gain insights into customer engagement in retail and to enable an intelligent traffic system in a city. However, current approaches to obtain this information are either invasive as they require to continuously run a dedicated smartphone app, or they compromise users’ privacy by sniffing the MAC addresses of their smartphones. As a consequence, lawyers, authorities, and the population are very skeptical toward adopting such innovative systems. We present DevCnt, the first system that counts in real-time the number of Wi-Fi enabled smartphones in a non-invasive manner while preserving by design the privacy of the smartphone users. This paper details how DevCnt detects active Wi-Fi scans performed by smartphones on a ZigBee device, and how DevCnt uses the number of detected scans to estimate the number of Wi-Fi enabled smartphones. Results from controlled and real-world experiments show that DevCnt: (i) detects more than 99% of active Wi-Fi scans even under interference from multiple wireless technologies, (ii) achieves up to 91% accuracy in the estimated smartphone counts, and (iii) provides meaningful estimates in a real test run involving hundreds of Wi-Fi transmitters.