Behavior change is a complex process in which people receive support in order to improve aspects of their behavior, for instance re- garding their health or lifestyle. Although there exist several theoretical approaches to model behavior change, including abstractions that can be applied to different use-cases, these are not easily translated into reusable components that can be integrated into implementable systems for per- suasion. This work discusses the need for an ontology-based approach to modelling interactions in eHealth systems, with the goal of achieving behavior change. This contribution includes an analysis of current mod- elling needs in behavior change, specially regarding: stages of change, motivation & ability factors, plans & actions, argumentation, and do- main modeling.