Work towards the development of a handheld health counseling agent designed to promote physical activity is described. Previous work on automated health counselors is discussed, along with the affordances of mobility and context awareness for health behavior interventions. We present a general-purpose software architecture for the rapid design and deployment of mobile health counseling agents. We also describe the results of an initial field trial in which such a mobile agent plays the role of an exercise coach designed to motivate users to walk more. Results were mixed. We found that the context awareness mechanism that was implemented for detecting walking led to greater user-agent social bonding, but less walking in study participants.