The authors review various definitions of panic and recent conceptual and empirical evidence that led to the revised definitions of panic in the Diagnostic and Statistical Manual of Mental Disorders. Fourth Edition (DSM-IV; American Psychiatric Association, 1994). The DSM-IV definitions of panic are more precise and more empirically based than those contained in the DSM-III-R, but are designed primarily for clinicians rather than researchers. Thus, it is possible that definitions of panic used in research under the DSM-IV system will encompass a heterogeneous mix of emotional phenomena. Recommendations on essential components of a conservative definition of panic are proposed for research purposes. Implications of this definition for limited symptom attacks and "non-fearful" panic are elaborated and related to final definitions of panic attacks and panic disorder in the DSM-IV.