Loneliness refers to the negatively perceived mismatch between one’s desired and actual engagement in social relations. Besides the individual suffering it implies, epidemiological evidence has suggested that loneliness is a critical determinant of mental health in late life. In this chapter, we provide definitions, operationalisations, prevalence estimates, and risk factors of loneliness in older people. Further, we summarise epidemiological evidence on the association between loneliness and depression, anxiety, suicide, and cognitive disorders and cover their potential underlying mechanisms. Despite an existing link between loneliness and mental health disorders, more evidence is needed to disentangle causality, bidirectionality and underlying mechanisms. Last, we discuss intervention strategies to reduce loneliness in older adults, focusing on their individual goals, challenges, and potential future development, as well as on the importance of inclusive communities and neighbourhoods. Interventions to facilitate social interactions and improve one’s social cognition seem promising yet challenging to implement and assess. Addressing loneliness remains a critical task to promote health in an ageing society.