Abstract: Systems and methods for tracking or viewing a user's physiological parameters before, during, and/or after physical activity. In some versions, a sensor device can be worn on a user's body and is configured to detect multiple physiological parameters and to wirelessly communicate with a mobile computing device. Optionally, the detected physiological parameters can be used to determine a selected set of work-out parameters, which can be concurrently displayed on the mobile computing device during the user's exercise/activity or after the exercise period. In some implementations, each of a plurality of users can wear a respective body-worn sensor device, and all of the sensors devices can wireless communicate with a designated mobile computing device for purposes of collectively (and concurrently) monitoring sets of physiological parameters for all of the users during the same workout, practice, or other activity.