This thesis deals with dual-diagnoses and the question which specific challenges may arise within professional social work with clients those apply to. An ever-growing number of patients with dual-diagnoses demonstrates the necessity of providing integrative and adjusted treatment concepts. The question why such integrative approaches are so essential and how these are set up, will be one of the main topics of this thesis. First, some terms and definitions will be discussed, as well as some selected diseases and their symptoms. Likewise, several models which deal with the connections between mental illness and substance-related dependence will be refered to. Furthermore, some specialities and difficulties concerning those clients will be addressed, based on the specific courses these clients' diseases tend to take. Based on that, modern integrative treatment concepts will be presented, that proved to be working for patients with dual-diagnoses. In a last chapter, the specific tasks of professional social work will be illustrated, as well as the special requirements for the cooperation with these clients.