The most important precursors of frequent offending were early antisocial and aggressive behavior; poor parental childrearing; criminal parents and siblings; social deprivation due to such factors as low family income, large family size, and poor housing; and low intelligence and achievement. The combining of the most important precursors to produce a prediction score was highly accurate in identifying frequent offenders. The causes of frequent offending can be determined by conducting prevention experiments which investigate the effectiveness of efforts to counteract the early precursors. Such research would reveal the extent to which early precursors were caused or only correlates. Experimental studies are also required to investigate the impact of criminal justice system responses on persons at high risk of becoming frequent offenders. Problems in measuring precursors are discussed. 61 references.