Along with environmental awareness, the interdisciplinary field of eco-feedback has become an active research interest in Human Computer Interaction (HCI), Computing, and Sustainable Design. While designers face the challenge of designing complex socio-technical systems (Norman and Stappers 2015), eco-feedback lacks comprehensive theoretical contributions to guide such efforts. This paper followed the PRISMA-P structured review method and reports on mostly empirical papers (N=36) on eco-feedback design from the past decade (2006-2016). The aim was to systematize the emerging field by classifying feedback types according to their impact on end users and identify theoretical grounding. Through our deep inductive engagement with the studies, we present four eco-feedback affordances: (1) Inform for awareness change; (2) Incentivize for motivation change; (3) Educate for behavioral change; and (4) Control for convenience. Current research is corroborating empirical evidence in support of eco-feedback, but their lack of theoretical grounding has resulted in incomparable constructs and inconsistent results.