Our environment is in constant flux, and in response, our emotions change dynamically (Kuppens & Verduyn, 2017). Thus, when seeking to influence our emotions, we must also dynamically tune our emotion-regulation efforts. This means that successful regulation does not involve merely using particular adaptive strategies, but rather, flexibly picking the right strategy to suit ever-changing situational demands (Bonanno & Burton, 2013). This means that emotion regulation researchers must understand flexibility, often defined as systematic variation in emotion regulation processes in synchrony with the context (Aldao et al., 2015). In this chapter, we first discuss variability as a precursor to flexibility, then introduce the components of emotion regulation flexibility, and finally discuss challenges and future research directions.