Several effective interventions have been developed for families with multiple problems (FMP), but knowledge is lacking as to which specific practice and program elements of these interventions deliver positive outcomes. The aim of this study is to assess the degree to which practice and program elements (contents of and structure in which care is provided) contribute to the effectiveness of interventions for FMP in general and for subgroups with child and/or parental psychiatric problems, intellectual disabilities, or substance use. We performed a quasi-experimental study on the effectiveness of practice and program elements provided in attested FMP interventions. Using self-report questionnaires, we measured primary (child's internalizing...