This entry defines fear of missing out (FOMO) as a construct based on negative affect, social comparison, and counterfactual thinking. Prior theoretical foundations of FOMO and the nature of the experience based on prior research findings are briefly summarized. Its relevance to social media and media in general is discussed with an overview of previous research in regard to problematic and compulsive social media and smartphone use, mental health and well‐being, and advertising and marketing. Last, FOMO over the average human life span is discussed, that is, commonalities and differences of the FOMO experience depending on peoples' age. This entry aims to provide a detailed overview of current research in FOMO and to explicate the construct in a detailed fashion. Further, its relevance to today's media environment, specifically social media, is discussed.