Background: Peer support is a mutual aid system based on the belief that someone who faced/overcome adversity can provide support, encouragement and guidance to those who experience similar situations. Objective: To conduct a systematic review that describes this concept and characterizes peer supporters, its practice and efficacy. Method: Research on ISI Web of Science, EBSCO Psychology and Behavioral Sciences Collection and Medline databases (from 2001 to December 2013) was conducted using as keywords "mental illness", "mental health", "psychiatric disability", "mental health services", combined with "peer support", "mutual support", "self-help groups", "consumers as providers", "peer-run services", "peer-run programs" and "social support". Results: We found 1,566 articles and the application of both the exclusion (studies with children, teenagers and elderly people; disease in comorbidity; peer support associated to physical illnesses or family members/caregivers) and the inclusion criteria (full text scientific papers, peer support or similar groups directed for schizophrenia, depression, bipolar or psychotic disorders) lead to 165 documents, where 22 were excluded due to repetition and 31 to incomplete text. We analyzed 112 documents, identifying as main peer support categories: characterization, peer supporter, practices and efficacy. Discussion: Despite an increasing interest about this topic, there is no consensus, suggesting realizing more studies.