Purpose of Review This review of the literature aims to explore the impact of the COVID-19 pandemic and the lockdown on teenagers’ mental health. We distinguish two groups: adolescents who had already been diagnosed with a mental disorder and the general population of adolescents. Recent Findings An increase in the number of mental health-related difficulties in adolescents has already been reported in previous health crises. Accordingly, the perceived well-being of teenagers declined during the COVID-19 pandemic. Adolescents with mental disorders were significantly more affected by the lockdown than those in the general population. Summary The effect of the COVID-19 crisis on the mental health of adolescents has been heterogeneous. The first pandemic wave was essentially associated with an increase of internalizing symptoms in adolescents, particularly anxiety, depression and eating disorders. The impact on externalizing symptoms was less clear, and seem to concern mostly adolescents with pre-existing behavioral disorders. During the second and later waves of the pandemic, an upsurge of suicidal ideation and attempts among adolescents have been reported in many countries.