This is a model of personal informatics systems that we hope would be valuable for research and development. It provides a common framework for describing, comparing, and evaluating the growing number of this class of systems. This model is composed of five stages (preparation, collection, integration, reflection, and action). These stages have four properties: barriers cascade to later stages; they are iterative; they are user-driven and/or system-driven; and they are uni-faceted or multi-faceted. From these properties, we recommend that personal informatics systems should 1) be designed in a holistic manner across stages; 2) allow iteration between stages; 3) apply an appropriate balance of automated technology and user control within each stage to facilitate the user experience; and 4) explore support for associating multiple facets of people's lives to enrich the value of systems.