This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: Our primary objective is to examine mental health and well‐being, mental health service usage, and adverse effects of Mental Health First Aid (MHFA) training on recipients of the intervention. We have three secondary objectives: 1. To examine the effects of MHFA training on recipients of the trainees' intervention, in terms of their knowledge about mental health and attitudes towards mental health problems. 2. To examine the effects of MHFA training on trainees' knowledge about mental health, attitudes towards mental health problems, number of encounters with people with mental health problems, and their own mental health and well‐being. 3. To examine the effects on organisations, looking at measures of absenteeism and productivity at work.