According to the regulations contained in the Social Services Act (SFS 2001:453), Swedish social services have a legal responsibility to provide support, care, and treatment for individuals with substance use problems.  This law mandate those who are responsible to provide treatment to motivate drug users to actively seek treatment on a voluntary basis, ensuring an end to their dependence on drugs. Studies have shown that although the treatment system largely focuses on promoting abstinence, about two-thirds of client’s relapse into substance use within one year after completing treatment. This dissertation focuses broadly on clients who repeatedly enter and use treatment for substance use disorders in the Swedish addiction treatment system. The aim of this thesis is to examine and identify the population groups who are repeated treatment users of the Swedish treatment system for substance use disorder, including both the voluntary treatment and compulsory care. This thesis was based on three national level databases. The results showed that clients with a higher degree of problems and problems in different areas of life also had an increased risk of having treatment for substance use disorder repeatedly. Clients who were older, men, reported more years of polydrug and alcohol use to intoxication, reported more compulsory care episodes for substance use, had ever been charged with crime, had ever been in inpatient mental health treatment, and had a higher ASI mental health symptom composite score, were significantly more likely to report more voluntary addiction treatment episodes. The strongest significant association with the number of treatment episodes was the number of compulsory treatment episodes for alcohol and drugs. Individuals who experienced prior compulsory care including mandatory treatment through LVU (law (1990:52)), been in prison, and had children mandated to out-of-home care, were more likely to have two or more entries in the compulsory care system for substance use disorder. In addition, this analysis showed that 59% of clients mandated to compulsory care dropped-out during their compulsory care episode, and that younger clients were significantly more likely to drop-out. Those who drop-out were significantly more likely to experience negative outcomes, i.e. additional sentence to compulsory care and higher risk of mortality.  A hierarchal logistic regression model also identified that individuals with riskier childhood conditions were more likely to have had repeated entries to compulsory care for substance use disorder. The indirect effects showed that a family history of substance use disorder and psychiatric problems are both associated with higher probability of institutional care as a child i.e. LVU, and that in turn, mandated childhood institutional care is related to repeated compulsory care intakes as an adult. Individuals who use treatment for substance use disorder repeatedly have a higher degree of problems i.e. an exposed and problematic group of individuals characterized by problem in several different areas of life. Growing up in a home environment with unfavorable conditions, mandated care before the age of 18 (LVU), compulsory care for substance use disorder as an adult, children taken into out-of-home care, and crime are the factors that are primarily associated with repeated treatment for substance use. A change in the view of treatment for clients in need of repeated use of treatment seems important, and access to adapted continuous care efforts are crucial to counteract the risk of relapse after a treatment episode of voluntary or compulsory care. Further, it seems important to motivate the client to complete the compulsory care without any deviation, since this seems to have positive effects on their substance use disorder.