Cell phones and other mobile devices have become part of human culture and are changing human activity and lifestyle patterns. Mobile phone technology continuously evolves and incorporates more and more sensors for enabling advanced applications. The latest generations of smartphones incorporate global positioning system (GPS) and wireless local area network (WLAN) location finding modules, vision cameras, microphones, accelerometers, temperature sensors, etc. The availability of these sensors in mass-market communication devices creates exciting new opportunities for data mining applications. In particular, healthcare applications exploiting built-in sensors are very promising. This chapter reviews different aspects of human activity recognition, including review of state-of-the-art technology, implementation, and algorithmic aspects.