Traditionally, medical adherence has been viewed as the extent to which a patient follows a prescriber's instructions. Adherence to treatment is a relevant factor able to influence results obtained by clinical trials and clinical practice. The components of adherence can be considered by the actions of patients, professionals, family and social support programs. Concerning mental health disorders, non-adherence is related to poorer treatment outcomes, such as lack of symptom stabilisation, homelessness, lower quality of life and hospitalisation. This book discusses clinical treatment outcomes, along with the clinical implications adherence has on patients with social anxiety, and chronic pain. It also provides several theoretical models of adherence.