The purpose of this dissertation is to evaluate the effectiveness and cost-effectiveness of Multisystemic Therapy (MST) in Sweden. This evaluation is set against the background of evidence-based social work practice and is organized around four separate but interrelated studies. The first of these studies used program theory reconstruction to investigate three conceptualizations of evidence-based practice (EBP). Here it is argued that evidence-based practice is not a homogeneous concept, and that interested parties within research, practice and policy may not have a shared vision of EBP, even though they may use the same terminology. The second study, a randomized trial, assessed the effectiveness of MST within the normally operating social services system for 156 youths who met the diagnostic criteria for conduct disorder. Youth were randomly allocated between MST and treatment-as-usual (TAU) groups. Assessments were conducted at intake and seven months after referral. Results from mulitagent and multimethod assessment batteries showed a general decrease in psychiatric problems and antisocial behaviors among participants across treatments. There were no significant differences in treatment effects between the two groups. The third study assessed the costs of treating conduct disorder with MST and TAU. From the perspective of the municipal social welfare system, all intervention costs were collected for the six-month period starting at randomization to treatment group. MST was not found to reduce the extent to which youth were placed outside of their homes. In addition, the costs of out-of-home placement were the same for both MST and TAU group youth. MST was, however, associated with a reduction in the use and costs associated with other non-placement services. This reduction was not found to offset the additional cost of MST. The fourth and final study investigated the treatment outcomes and costs associated with MST versus TAU for intervention with substance abusing and non-substance abusing conduct disordered youth. This study found no differences in treatment outcome between these two groups. This dissertation found MST to be equally effective but less cost-effective than TAU.