Loneliness and social isolation, prevalent among older adults, have profound and often interconnected effects on physical and mental health, leading to increased mortality, depression, and cognitive decline. This paper reviews the literature on social isolation and loneliness in seniors. Key factors contributing to social isolation include the death of a spouse, physical relocation, retirement, changes in family structures, changes in mobility, chronic health problems, household income, race, ethnicity, gender, and sexual orientation. These factors have varying impacts on the social well-being of older adults. Research suggests that subjective social isolation has a stronger link to mental health, while objective social isolation is more strongly related to physical health. Both social isolation and loneliness are associated with higher rates of depression and psychological distress among seniors. Understanding social isolation and loneliness in seniors is crucial for developing targeted interventions and support systems to enhance the well-being of this growing demographic.