In recent years, there has been explosive growth in smartphone penetration. Today, more than 90 percent of the worldÕs population is covered by cellular networks, and nearly one quarter of the population uses smartphones. 1 This growth is set to continue as inexpensive devices become available and mobile phone users switch over to smartphones. This makes smartphones an ideal vehicle for large-scale sensing systems, because while a PC sitting on a desk doesnÕt know much about its user, a phone accompanying its user can learn about the user through a range of embedded sensors. The persistent connectivity offered by smartphones also lets users access information from almost anywhere.Combining these two capabilities together can lead to interesting applications, especially when the user is away from home or work, without access to traditional devices. For example, consider a user traveling by car. He or she might wonder, ÒAre there any disruptions on my regular route that might cause delays? Will I be able to find parking when I arrive?Ó Furthermore, he or she might want to automatically send a message to someone saying, ÒIÕm running late due to traffic.Ó Smartphones can help, enhancing the driving experience.In particular, we developed two smartphone-based prototype systems that address this scenario. Our first system, ParkSense, can sense on-street parking-space occupancy when coupled with electronic parking payment systems. Our second system can sense and recognize a userÕs repeated car journeys to provide personalized alerts to the user. Both systems aim to minimize the impact of sensing tasks on the deviceÕs lifetime so the user can continue using his or her smartphone for its primary purpose.