Given the central role of panic attacks in the diagnosis of panic disorder, an adequate measure of panic attacks is essential. Panic frequency is routinely assessed either by simply asking individuals to estimate the number of panic attacks experienced during a given time in a questionnaire or assessment interview or by having them continuously self-monitor. Panic frequency obtained by such methods is unreliable and invalid or time-consuming, respectively. The purpose of this project was to investigate the reliability and validity of a Panic Attack Frequency Calendar (PAFC), modeled after a time-line follow-back (TLFB) procedure (e.g., Sobell & Sobell, 1979) that has been used for years to reliably and validly assess daily alcohol use through self-report over extended periods of time. The participants consisted of 74 adult individuals (ages 18-57) who indicated that they had experienced a panic attack within the past two weeks. Participants completed a battery of self-report questionnaires, including a retrospective frequency measure, and administered an 8-week PAFC. Participants were then randomly divided into either a self-monitoring group that recorded information in a diary about any panic attacks that they experienced over the following two weeks or a non-self-monitoring control group. All participants were administered another retrospective frequency questionnaire and a 10-week PAFC two weeks after the administration of the first PAFC. Reliability was determined from the two-week stability estimates across the PAFCs for the eight-week period that overlapped both assessments. This was done with several composite panic behavior variables; daily and weekly test-retest reliabilities were also calculated. Concurrent validity was established by comparing panic frequency from the PAFC with that obtained from the diary and the retrospective frequency measure. Further validity was established via correlating the PAFC with the self-report questionnaires. Results are discussed in light of their implications for the assessment of panic attacks. / Master of Science