Background: Although emotion is central to most models of children's well-being, few studies have looked at how well-being is related to the ways in which children regulate their emotions. Aims: The aim of this study was to examine the associations among children's emotion regulation strategy choice and their emotional expression, behaviour, and well-being. The study also investigated whether contextual factors influenced the emotion regulation strategies children chose to use. Sample: Participants (N = 33) were selected from four Year 5/6 composite classrooms situated in low-socioeconomic urban communities in New Zealand. Method: Questionnaires were used to measure children's well-being and teacher-reported emotional and behavioural problems. Emotional expression and emotion regulation strategies were measured through video-recorded observations in the classroom. A total of 1,184 instances of emotion regulation strategy use were coded using a framework based on Gross' process model of emotion regulation. Results: The findings highlight the complexity of the relations among emotion regulation, emotion expression, and well-being. Some strategies, such as "Cognitive Reappraisal," were effective at upregulating negative emotion in the short term, yet not strongly associated with well-being. Others, such as "Situation Modification: Physical," were positively associated with well-being, yet not with an immediate change in a child's emotional experience. The findings also suggest children flexibly use different strategies in relation to different contextual demands. Conclusion: These findings may be used to guide future intervention efforts which target emotion regulation strategy use as well as those which focus on teachers' support of children during emotionally challenging situations.