The average daily prison population in England and Wales has grown from less than 10,000 in 1940 to almost 45,000 in the first months of 1980. Four-fifths of this population consists of people deliberately sent there by the courts following conviction, and it is among these people that reductions must come if the role of prisons is to be reduced. Although longer sentences appear to be prime factor in the increased prison population, prison statistics show that the growth in the numbers of persons found guilty and therefore becoming available for sentencing has been a greater factor in the growth of prison populations. However, the courts have reduced the proportion of offenders sent to prison and mete out long sentences no more often than they were used 10 or 20 years ago. The enormous rise in prison population can be accounted for almost entirely by the increased number of prosecution of those under 21 years old. From 1955 to 1976, the proportion of the prison population under 21 years grew from 34 to 48 percent. Therefore, it seems reasonable to argue that the factors in the processes of selection between crime and imprisonment that account most for the growth and juvenilization of the prison population have to do with the role of the police in detecting and prosecuting offenders: thus the question as to what extent young people have become more criminal or merely more often caught becomes crucial. Tables and 40 references are supplied.