BackgroundIncarcerated mothers are a marginalised group who experience substantial health and social disadvantage and routinely face disruption of family relationships, including loss of custody of their children. To support the parenting role, mothers and children’s units (M&Cs) operate in 97 jurisdictions internationally with approximately 19 000 children reported to be residing with their mothers in custody-based settings.AimThis rapid review aims to describe the existing evidence regarding the models of service delivery for, and key components of, custodial M&Cs.MethodA systematic search was conducted of four electronic databases to identify peer-reviewed literature published from 2010 onwards that reported quantitative and qualitative primary studies focused on custody-based M&Cs. Extracted data included unit components, admission and eligibility criteria, evaluations and recommendations.ResultsOf 3075 records identified, 35 met inclusion criteria. M&Cs accommodation was purpose-built, incorporated elements of domestic life and offered a family-like environment. Specific workforce training in caring for children and M&Cs evaluations were largely absent. Our systematic synthesis generated a list of key components for M&C design and service delivery. These components include timely and transparent access to information and knowledge for women, evaluation of the impact of the prison environment on M&C, and organisational opportunities and limitations.ConclusionThe next generation of M&Cs requires evidence-based key components that are implemented systematically and is evaluated. To achieve this, the use of codesign is a proven method for developing tailored programmes. Such units must offer a net benefit to both mothers and their children.