Adolescence is a developmental period with mental health vulnerability. The maturation of the nervous system together with the increase in daily stressors, makes this period challenging. Psychosocial stress, in particular, is salient for adolescents as they move away from parents towards their peers. Therefore, there is a need for early interventions that support managing of stress and the emotional turmoil, that is specific to this stage of human life. Mindfulness based interventions (MBI) show promising results in adults with its positive effects on mental wellbeing. In adolescents, the effects are not clear, however, research is expanding. The investigation on neural activity related to mindfulness in adolescents is scarce. This thesis investigates the effects of MBI on young adolescents with an emphasis on brain activity towards psychosocial stress, which is studied under a large randomized controlled trial, the Mindfulteen Study. We find that psychosocial stress effects adolescents with similar stress related brain activity as adults. Positive social evaluative feedback, however, engages regions such as right frontal pole related to motivation and superior parietal lobule suggesting a broadening of attention. Negative feedback, on the other hand, engages self-referential regions such as the posterior cingulate cortex, only in young adolescents who present depressive symptoms. MBI does not significantly improve clinical scores overall, only a subgroup of individuals with previous depressive episodes show a change in the expected direction. However, we observe differences in how the brain reacts and how it processes social feedback. In the intervention group, receiving positive feedback activates subcortical regions related to motivation such as the putamen and cortical regions related to executive processes, such as the dorsolateral prefrontal cortex (DLPFC) compared to control group. The intervention group also shows less self-referential, ruminative state, called the default mode network (DMN) after MBI in all conditions. The relationship of brain activity to clinical scores changes dramatically, with relation to trait mindfulness scores. Even though there are no immediate beneficial clinical effects of MBI on young adolescents, there is a change in brain activity and brain dynamics in the reception and recovery from psychosocial stress. The relationship between this change with clinical scores is switched between two time points, signalling a possibility of increased effects on those who present low 9 indices of wellbeing, which need to be investigated and confirmed longitudinally. Reasons for lack of immediate effects and suggestions for developmental stage adaptation are discussed.