Nowadays, smartphones are ubiquitous and becoming more and more sophisticated, with ever-growing computing, networking, and sensing abilities. This has been changing the landscape of people’s daily life and has opened the doors for many interesting applications, ranging from health and fitness monitoring, personal biometric signature, urban computing, assistive technology, and elder-care to indoor localization and navigation, and so on. Besides the inclusion of sensors, such as accelerometer, compass, gyroscope, proximity, light, global positioning system (GPS), microphone, camera, the ubiquity and unobtrusiveness of the phones and the availability of different wireless interfaces, such as Wi-Fi, 3G, and Bluetooth, make them an attractive platform for human activity recognition (HAR) [1–3].