Personal Informatics (PI) systems allow users to collect and review personally relevant information. The purpose commonly envisioned for these systems is that they provide users with actionable, data-driven self-insight to help them change their behavioral patterns for the better. Here, we review relevant theory as well as empirical evidence for this ‘Self-Improvement Hypothesis’. From a corpus of 6568 only 24 studies met the selection criteria of being a peer-reviewed empirical study reporting on actionable, data-driven insights from PI data, using a ‘clean’ PI system with no other intervention techniques (e.g. additional coaching) on a non-clinical population. First results are promising—many of the selected articles report users gaining actionable insights—but we do note a number of methodological issues that make these results difficult to interpret. We conclude that more work is needed to investigate the Self-Improvement Hypothesis and provide a set of recommendations for future work.