In the field of mental health, assessing quality of life may impact clinical care and policy decision-making as it indicates treatment efficacy. Despite the wide range of definitions of quality of life concept within the research field, quality of life instruments may be useful in quantifying and reducing the societal burden of mental disorders and planning clinical care at the individual level. The current chapter presents the most used theoretical frameworks and quality of life measures in mental health research. In addition, the findings of recent systematic reviews and meta-analyses on quality of life levels and effects of treatments on quality of life in adult patients are synthesized for several common and severe mental disorders. Research has shown that people with depression, anxiety disorders, posttraumatic stress disorder, and psychosis experience significantly lower quality of life than healthy controls, and have deficits in various quality of life domains. The results of systematic reviews and meta-analyses provide consistent support for the beneficial effects of psychological interventions on improving quality of life in individuals suffering from anxiety and depression. A manualized emotion regulation-based transdiagnostic intervention improved quality of life and functional impairment in individuals with internalizing disorders at post-intervention and six-month follow-up. In people with schizophrenia, more effort is needed to optimize psychological interventions in order to improve their quality of life. Future randomized clinical trials might include more quality of life measures and combine mindfulness-based stress reduction with physical exercise and other promising modern interventions to enhance their efficacy in improving patients’ quality of life.