Psychiatric mother-baby units (MBUs) are currently viewed as best practice, particularly in the UK, Australia and France, for improving outcomes for mothers and babies when the former are experiencing severe forms of mental illness. A growing number of publications have examined MBUs, but to date, there has not been a comprehensive review of these studies. As such, the systematic review reported in this paper sought to address this gap. A systematic search was conducted for peer-reviewed research and grey literature published in English between 2000 and 2015. A final sample of 44 publications were identified that reported on empirical findings with regard to MBUs. Three quarters of the studies focused on individual MBUs and most studies were quantitative. A thematic analysis of the studies identified three major themes: (1) admissions data, (2) outcomes for mothers, and (3) programmes and interventions. The analysis also identified four secondary themes: (i) follow-up after discharge, (ii) separation of mothers and babies after discharge, (iii) client satisfaction with MBUs, and (iv) partners of women admitted to MBUs. The findings of the review highlight gaps in knowledge about MBUs and provide suggestions for future research.