The two broad goals common to State correctional agencies are the protection of public safety and the provision of basic care. Classification is most concerned with the safety of the public, staff, and inmates. In addition, classification is the theoretical means used by a correctional system to assess inmates and departmental resources along the same dimensions. Classification design may involve either a descriptive of prescriptive approach. The descriptive approach codifies and formalizes the current decision patterns within the organization. The prescriptive approach, used here, requires that the agency address important issues, such as the requisite technical skills of the staff, accuracy of agency data bases, and agency commitment to classification system implementation. The four classification system goals were used to develop an appropriate system, to place inmates in the lowest level of security classification while protecting the public safety, to influence institutional programming through more effective resource allocation, and to improve management and service delivery. Classification should be based upon verifiable assessment of individual likelihoods towards such in-prison factors as violence, assaultiveness, security risk, and alcohol and drug use and such post-release factors as social instability, violence, unemployment, alcoholism, and recidivism. The theoretical assumption underlying the Illinois classification system is that such systems are concerned with two types of maladjusted behaviors, instability and violence. About 55 references and sample forms are appended. Footnotes and illustrations are supplied.