Internalizing disorders, which include depression and anxiety as well as several other syndromes, are common and impairing conditions that form one of the spectra in structural models of psychopathology (i.e., the Hierarchical Taxonomy of Psychopathology). This chapter reviews the hierarchical structure and contents of the Internalizing spectrum, as well as their key features and challenges that commonly arise when assessing these symptoms. Four types of assessment tools are presented—self-report measures, clinical interviews, ambulatory assessment, and performance-based clinician-rated measures (i.e., behavioral avoidance tasks)—along with their strengths and limitations, applications, and example measures as they relate to the Internalizing spectrum. The chapter also discusses considerations for assessment of internalizing symptoms in diverse populations and future directions in this area.