This paper presents a review of the literature on mass effects and their importance in choice behavior, i.e. how individual behavior is influenced by the behavior of others, with the goal of extracting lessons for transportation policy. An overview of psychological theories explaining the process underlying mass effects and conformity behavior is given. This is followed by a presentation of evidence of mass effects on choice behavior both outside and within transportation planning, covering contexts ranging from long term choices such as residential location to short term decisions such as driving behavior. Based on this review, modeling approaches for studying mass effects and their data requirements are then synthesized, highlighting the advantages and limitations of each. The paper concludes with a discussion of the importance of leveraging the power of mass effects for designing transportation policies aimed at promoting sustainable and safe mobility, and of challenges for future work in this area.