An activity tracker is an electronic device or app that measures and collects data about an individual's movements and physical responses, towards the goal of monitoring and improving their health, fitness or psychological wellness over time.
Activity trackers are a more sophisticated version of the pedometer; in addition to counting steps, they contain additional sensors such as accelerometers and altimeters to collect (or estimate) measures including the speed and distance travelled, heart rate, calorie expenditure, or the duration and quality of sleep.
Improvements in computing technology since the 1980s, recently driven by the rapid advancement of smartphones, paved the way for the spread of wearable tracker devices with integrated sensors. Large amounts of sensor and user-input data is synced with mobile apps, much of it sensitive in nature, such as fitness, mood, sleep, water intake, medicine usage, sexual activity, menstruation, and potential illnesses. This has led to privacy concerns around how consumer information is stored and analyzed by the companies involved.

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