Mental health treatments, including antidepressant treatment such as Selective Serotonin Reuptake Inhibitors (SSRIs) and Cognitive Behavioural Therapy (CBT) are effective interventions for depression and anxiety in young people. However, the short and long term effects of one or both of these forms of treatment are unknown in longitudinal population studies. There remains a paucity in how these treatments work, what combination of treatments work, what works best for whom, whether some symptoms are treated better than others and how variable mental health responses are following treatment. We will use data recently collected in ALSPAC on mental health treatments and examine mental health responses to address those research gaps. We will develop a longitudinal research tool which highlights these results which will be made freely available to researchers to apply to other research questions.