This chapter describes conceptual and methodological issues in the assessment of psychopathology, along with substantive results. It considers different types of assessment instruments, including structured interviews, brief self-rated and clinician-rated measures, projective techniques, self-report personality inventories, and behavioral assessment and psychophysiological methods. The chapter reviews research on the validity of assessment instruments and research on clinical judgment and decision-making. Treatment utility describes the extent to which an assessment instrument contributes to decisions about treatment that lead to better outcomes. Behavioral assessment methods and psychophysiological assessment can provide valuable information. In contrast to the dispiriting findings concerning the value of clinical experience for personality assessment judgments, the research literature supports the value of training for some judgment tasks. Scientifically minded practitioners need to be aware of the phenomenon of illusory correlation, which suggests that clinicians can be convinced of the validity of assessment indicators in the absence of validity.