Study data were based on 420 recorded cases of juvenile detention from all jurisdictions in the country between November 1978 and the end of 1979. Analysis indicated that detention was imposed especially on violent offenders, both juvenile and adult. Those classified as habitual recidivists, individuals likely to skip town, and persons with bad reputations were commonly detained. Preventive detention was scarcely used until the 1960's, when it increased markedly. In the 1970's the practice decreased again due to the passage of a new law stipulating complex conditions for the incarceration of minors. The legal prescriptions are imprecise, conflict with procedural law, and are not clearly separated from correctional measures. The prevention of criminal acts was the most frequently cited reason for juvenile detention. Maps and tabular data are given. Additional study data are appended and a 31-item bibliography is provided.