The purpose of this eBook is to overcome a key barrier for the dissemination of psychological interventions into practice, by supporting the public accessibility of evidence-based treatment manuals and implementation guides for comorbid mental and substance use disorders. Our intentions are twofold. First, to facilitate further research and replication of comorbidity treatment models internationally, thereby growing and strengthening the field and improving what we 'know'. Second, to enable clinicians across primary, secondary, and tertiary services to implement high quality, theoretically grounded, and empirically-supported psychosocial treatments with people experiencing mental and substance use disorders, thereby improving what we 'do'. This eBook brings together a collection of papers that describe the treatment procedures for comorbid presentations encountered in diverse clinical settings. The diversity of treatment approaches described reflects the very nature of comorbidity: where the combination of heterogenous diagnostic, demographic and contextual features can complicate treatment. Furthermore, these papers acknowledge and address the unique challenges of working in outpatient, inpatient and custodial settings.