Life span developmental theories suggest that as individuals age, they accumulate knowledge about how to deploy emotion regulation (ER) strategies effectively and learn how to match their ER strategy use with changes in situational demands. Using an event-contingent experience sampling design wherein 150 adults Age 18 to 89 years reported on 64,213 social interactions (M = 427.41, SD = 145.66) during 9 weeks of daily life, this study examines (a) age-related differences in individuals' usual ER strategy use (reappraisal, suppression) during everyday social interactions, (b) age-related differences in how much individuals' use of these two strategies varies across social situations-ER variability, and (c) age-related differences in the extent to which ER strategy use covaries with relational (close vs. nonclose others) and emotional (happy, sad) contextual features of those social situations-ER flexibility. In line with a small body of prior work, usual ER strategy use did not differ across adulthood and ER variability was lower at older ages. Results from multilevel models of intraindividual covariation suggested that individuals flexibly matched their ER strategy implementation to changes in emotional context-especially when interacting with close others. The results also provided evidence that the intraindividual covariation between relational context and use of suppression was weaker at older ages. Beyond these specific findings, this study demonstrated the utility of experience sampling designs, event-contingent reports, and the measurement/modeling of intraindividual variation and covariation for study of emotional development across the life span. (PsycINFO Database Record (c) 2019 APA, all rights reserved).