Approximately 13% of people worldwide are affected by mental health disorders, that cause substantial reductions in quality of life. Unfortunately, most medications for mental disorders suffer from two major problems: 1) there is a strong heterogeneity in their efficacy; 2) adverse side effects are relatively common. Therefore, specific biomarkers or tools allowing more insight into underlying mechanisms should be developed, which could be used to identify correlated biological measures which could aid patient stratification and thereby improve the efficacy and reliability of medications, and/or help to develop better ones. In this thesis, magnetic resonance imaging (MRI) techniques were used to study functional and structural changes in the brain induced by different external stimuli. These included: taxing working memory, emotional tasks, stimulant medication and an exercise intervention. MRI techniques were applied to measure the influences on functional reactivity and brain connectivity, on brain metabolite concentrations and on volumetric measures of specific subregions, cerebral blood flow and cerebral blood volume of the brain. Additionally, measures of behavior, cognition and peripheral physiology were applied and correlated to neurobiological, MRI-based changes. The overall aim of this thesis was, by using comprehensive MRI studies, to investigate influences of specific cognitive, pharmacological, and physical interventions on selective behavioral, cognitive and neuronal changes of the brain. We here; 1) investigated cognitive task-based interventions on specific brain functions, and next examined medication effects on certain symptom clusters and brain functions of individuals with ADHD, and 2) investigated effects of a longitudinal exercise intervention on various measures of brain plasticity in young healthy adults.