Loneliness is conceptualized as a dissatisfaction with the quality of one's social relationships, reflecting the human need to belong. During the COVID-19 pandemic, Governments imposed strict measures to reduce contagious, including school closures, physical distancing, and home quarantine/lockdown. The resulting reduction of social contacts and consequent change of daily routines increased experiences of loneliness that, in turn, exacerbated mental health difficulties/internalizing problems. This was particularly the case among adolescents, for whom interactions with peers play such a prominent role in overall wellness. However, relatively few researchers investigate the mechanisms linking loneliness and mental health during the COVID-19 pandemic. The current research project aims to better understand these issues during the COVID-19 pandemic in a large sample of Italian adolescents ages 14-19 years. Specifically, we aim to explore: (1) the prevalence of loneliness and mental health difficulties/internalizing problems; (2) the impact of loneliness on mental health; and (3) the role of potential protective and risk factors in the links between loneliness and aspects of mental health. The participants will complete an online survey to assess the experience of loneliness, mental health difficulties (e.g., anxiety, depression, sleep-related problems), and protective (e.g., support by peers), and risk (e.g., social media usage) factors during the COVID-19 pandemic. The current research project will help provide teachers and school principals with important tools to create a supportive and secure school environment during the uncertain period of the COVID-19 pandemic.