This study aimed to fill a void in the research regarding police behavior by focusing on the formation and creation of cognitive suspicion by officers. The study also examined formal actions (stops) taken by the police pursuant to that suspicion. The study was conducted using observational research methods and collected quantitative and qualitative data on officer suspicion. Data were collected by observers who rode along with patrol officers from April 2002 to November 2002. Field observers used three major data collection instruments in order to gather as much relevant information as possible from a variety of sources and in diverse situations. The Officer Form was an overall evaluation of the officer's decision-making characteristics, Suspicion Forms captured information each time an incident occurred, and a Suspect Form was a compilation of data from the citizen who had the encounter with the officer. Additional documents included informed consent forms, a card detailing the language to be used for the initial contact with citizens, and hourly activity forms. Anytime a suspicion was formed or a formal action was taken after a suspicion was formed, the observer debriefed the officer as to his or her thoughts and elicited the officer's overall rating of the encounter. Data in this collection include general demographic characteristics of the officer and the suspect, as well as the area in which the suspicion was formed. Data was also gathered regarding what led the officer to form a suspicion, and why a person was or was not stopped.