This volume is a wide-spectrum reflection springing from the contributions of some of the most important European and American researchers in the dual diagnosis field, who were involved in the national SIP.Dip. Conference in Milan on July 2002. They contributed to a shared understanding of issues such as the relevance of psychiatric diagnosis in addiction treatment planning, with experiences from Germany, Netherlands, Greece, Spain, England, Italy, and Thomas Mc Lellan's paradigmatic research on assessment instruments carried out over the last 30 years in the USA.