The purpose of my study is to investigate the single and joint effects of a mindfulness education program for children (MIndUp) and a mindfulness-based stress reduction program for teachers (SMART-in-Education) on elementary school children’s social and emotional well-being, classroom behaviors, and stress reactivity, and academic achievement. This study will contribute to the question of whether and how much experience with mindfulness practices on behalf of the adult is necessary to successfully implement a mindfulness program with children. As part of this study, teachers and children in their classrooms are randomly assigned to 4 conditions: 1. Children receive MindUp and teachers receive SMART, 2. Children receive MindUp (no program for the teacher), 3. Teacher receives SMART (no program for the children), 4. Neither teacher nor children receive a program. The child-outcome variables of interest address multiple levels of development, namely biological, social and emotional, and cognitive development. Variables of interest include salivary cortisol, self-, peer- and teacher-reported social and emotional well-being, classroom behaviors, peer relationships, and grades at the end of the school year. Furthermore, we will assess implementation fidelity for both, the MindUp and the SMART program.