People spend considerable time on digital media, and during this time they are often exposed to others’ emotion expressions. This exposure can lead their own emotion expressions to become more like others’ emotion expressions, a process we refer to as digital emotion contagion. This paper reviews the growing literature on digital emotion contagion. After defining emotion contagion, we suggest that one unique feature of digital emotion contagion is that it is mediated by digital media platforms that are motivated to upregulate users’ emotions. We then turn to measurement, and consider the challenges of demonstrating that digital emotion contagion has occurred, and how these challenges have been addressed. Finally, we call for a greater focus on understanding when emotion contagion effects will be strong versus weak or non-existent.