**Michel C. A. Klein, Adnan Manzoor and Julia S. Mollee \***

Department of Computer Science, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; m.c.a.klein@vu.nl (M.C.A.K.); a.manzoor@vu.nl (A.M.)

**\*** Correspondence: j.s.mollee@vu.nl; Tel.: +31-20-598-7743

Received: 5 May 2017; Accepted: 13 June 2017; Published: 19 June 2017

**Abstract:** Lack of physical activity is an increasingly important health risk. Modern mobile technology, such as smartphones and digital measurement devices, provides new opportunities to tackle physical inactivity. This paper describes the design of a system that aims to encourage young adults to be more physically active. The system monitors the user's behavior, uses social comparison and provides tailored and personalized feedback based on intelligent reasoning mechanisms. As the name suggests, social processes play an important role in the Active2Gether system. The design choices and functioning of the system are described in detail. Based on the experiences with the development and deployment of the system, a number of lessons learnt are provided and suggestions are proposed for improvements in future developments.

**Keywords:** e-coaching; m-health intervention; personalization; healthy lifestyle; physical activity
