Following a summary of methods and definitions used in the study, this report discusses problems that complicated probation reporting, such as differences in requirements for reporting probation cases to a central agency among the 50 States, the District of Columbia, and Puerto Rico. The total probation population as of December 31, 1979, is estimated, and data on entries and removals between 1977 and 1979 are used to predict a trend toward increasing use of probation. Breakouts for the same period on felonies and misdemeanors show that probation cases are almost evenly divided between the two categories. The Uniform Crime Reports, National Prisoner Statistics, Uniform Parole Reports, the Census of Jail, and this study provide the basis for an analysis of regional variations in the rates of jail, prison, and conditional release and the use of probation in relation to these indicators. In the North Central region and the South, prison use is higher and probation lower than the national averages, while the Northeast and West have lower proportions of prisoners and higher probation rates. The survey of agency workloads considers cases supervised other than probation, presentence reports, and levels of supervision. The average national caseload was calculated as 66. The study concluded that a national aggregate probation data reporting system was feasible. The appendixes contain statistical tables, State-by-State explanations of exceptions to reporting criteria, lists of States showing their probation structure and reporting bases, and five references.