Loneliness, a subjective and multifaceted emotional experience, has long been recognized as a critical factor in mental and physical health. The COVID-19 pandemic brought this issue into sharper focus by globally disrupting social routines and amplifying conditions conducive to social disconnection. This chapter examines how loneliness evolved before, during, and after the pandemic, drawing on empirical data, theoretical perspectives, and a systematic review with meta-analysis. It begins by outlining pre-pandemic loneliness trends, emphasizing age-related vulnerabilities and structural contributors such as mobility, single-person households, and the ambivalent role of digital communication. This chapter then addresses the immediate psychological and social effects of pandemic-related restrictions, highlighting both universal and group-specific experiences of fear, grief, and disruption. A meta-analysis synthesizing longitudinal data from diverse populations reveals a small but statistically significant global increase in loneliness. Subsequent sections explore how this change was shaped by individual differences, technological access and usage, and shifting societal norms. Younger individuals and those with preexisting mental health issues or personality pathology were disproportionately affected, while some older adults exhibited resilience. This chapter concludes by reflecting on the pandemic’s potential to reduce the stigma around loneliness and reshape public discourse. It argues that studying loneliness in this context offers actionable insights into societal vulnerability, resilience, and the need for sustained public health strategies that address social as well as physical well-being.