This brief is one in a series exploring issues related to the implementation of evidence-based interventions. It defines contextual fit, which is based on the premise that the match between an intervention and local context affects both the quality of the intervention implemented and whether the intervention actually produces the outcomes desired for the children and families receiving the intervention. An operational definition, formal measures, and systematic research that guides both policy and practice are needed before assessing the fit of evidence-based interventions for a particular context can become common practice. The brief describes eight elements that combine to establish the fit between an intervention and a setting. It also includes recommendations for developing formal measures of contextual fit. This brief is one of three developed as part of the Investing in What Works project, which builds the knowledge and supports that evidence-based programs can use to improve the quality and outcomes of interventions funded through federal investments.