There is growing interest in school-based programs to promote students' subjective well-being (SWB). Students with greater SWB tend to have stronger relationships with their teachers and classmates, as well as behave in more positive ways. Drawing from theory and research pertinent to promoting children's SWB, we developed an 11-session classwide positive psychology intervention that targeted elementary school students' novel use of character strengths, gratitude, kindness, and relationships in the classroom. Throughout a pilot study, children in fourth grade experienced clinically meaningful lasting gains in multiple indicators of SWB, particularly positive affect and satisfaction with self, but no changes in distal indicators of behavioral student engagement (attendance, office disciplinary referrals). This initial application by school psychologists partnering with a classroom teacher provides evidence of promise that elementary school children can benefit from participation in universal positive psychology interventions that target internal assets (gratitude, kindness, and signature strengths) and environmental resources (student-teacher and peer relationships).Keywords Elementary school children . Positive psychology interventions . Classwide implementation . Subjective well-being Mental health is increasingly viewed as a complete state of being, consisting not merely of the absence of psychopathology, but also the presence of positive indicators of subjective well-being (SWB; Keyes 2009). Within a positive psychology approach, SWB is often used as an indicator of the complete range of psychological functioning, from miserable to content to flourishing. Researchers interested in happiness often define it as SWB, a multidimensional construct conceptualized as having cognitive and affective aspects (Diener et al. 2009). The cognitive aspect is indicated by life satisfaction judgments-global appraisals of the quality of one's life overall or in regard to specific domains of life. The affective aspects involve how often a person experiences positive emotions (e.g., delight, pride, and zest) and negative emotions (e.g., sadness and anger). People with high SWB judge their life to be going well on the whole and experience more frequent positive than negative emotions on a daily basis. Studies with children (Greenspoon and Saklofske 2001) and adolescents (Suldo and Shaffer 2008) find that youth with the best academic, social, and physical health outcomes have both minimal symptoms of psychopathology and high SWB (i.e., complete mental health). Thus, comprehensive mental health services entail both the prevention and amelioration of psychopathology, and the promotion of SWB. However, the science of how to increase SWB through school-based intervention strategies pales in comparison to knowledge of how to treat psychopathology.As explicated in a 2013 joint report from the Institute of Education Sciences (IES) and the National Science Foundation (NSF), the process of developing an educational intervention generally st...