This is a protocol for a Cochrane Review (Methodology). The objectives are as follows: Our objectives are to assess the beneficial effects and adverse events of placebos, usual care, and wait-list interventions for all mental health disorders. There are three research questions: What are the beneficial effects and adverse events of: pharmacological, psychological, and physical placebos versus no-treatment or wait-list interventions; usual care versus no-treatment or wait-list interventions; and wait-list versus no-treatment interventions?