This paper presents findings of a study that examined Missouri high school principals' perceptions of the effectiveness of various disciplinary procedures, with a focus on in-school suspension (ISS). A survey mailed to 200 Missouri high school principals elicited 159 responses, a 77 percent response rate. The questionnaire asked principals to identify the disciplinary procedures currently in use at their schools and to specify under which conditions they perceived each measure to be most effective. Inschool suspension (ISS) was used by 88 percent of the respondents, who perceived it to be the most effective sanction for serious incidents that disrupted the school environment. ISS was limited to behavior that, although hostile and threatening, was not significantly physical, violent, dangerous, and/or illegal. Smoking in the rest room or truancy are examples of such behavior. Although the principals rarely utilized corporal punishment, they perceived out-of-school suspension (OSS) as an effective measure for more serious incidents, such as oncampus possession of alcohol or marijuana, fighting, extortion, being under the influence of alcohol or drugs, obscene language toward a teacher, and physical sexual harassment. OSS was utilized by 98 percent of the sample. One table is included. (LMI)