This chapter presents a framework for the assessment and diagnosis of substance use disorders (SUDs) with an emphasis on integrating a dimensional approach into the categorical system that historically underlies established diagnostic interviewing tools. Supplementing categorical diagnoses with dimensional tools allows for a more nuanced clinical picture than simply determining whether an individual meets the criterion for an SUD. Additional categories reviewed include substance use frequency, degree, and function, associated impairment, and co-occurring psychological symptoms. The goal of a dimensional assessment is to develop an individualized treatment plan that targets the functional underpinnings of substance use and associated psychological symptoms. We present guidelines and tools to achieve this as well as two case examples. This chapter can be used as a clinical tool for the assessment of substance use at screening, treatment referral, and treatment planning, encompassing a broad scope beyond the traditional categorical diagnosis.