The aim of this integrative literature review was to identify high quality empirical research and theoretical literature on the sources of meaning in life and people's beliefs regarding the meaning of life. This will inform current mental health clinical practice and research by providing a synthesis of empirical and theoretical literature. Failure to address meaninglessness or the existential crisis can lead to psychopathologies such as depression, anxiety, addiction, aggression, hopelessness, apathy, lower levels of well-being, physical illness, and suicide. Integrative literature reviews incorporate empirical research and theoretical literature. The inclusion criteria were primary research and theoretical papers and books by prominent theorists. Thirty-nine items underwent the critical appraisal process. Thirty-two papers or books were included. Overwhelmingly the results revealed that relationships, particularly relationships with family, are cited as the most important source of meaning in people's lives in all cultures and age groups. There was no consensus identified to answer the meaning of life question. These results and future research will allow mental health clinicians to help patients deepen their understanding of themselves, identify where they find meaning and understand their beliefs about meaning of life, contributing to a reduction in symptomatology and meaninglessness, and an increase in happiness, life satisfaction, positive affect, better coping, psychosocial health and well-being, and more meaningfulness in life.