Correctional influence on juvenile convicts is one of the priorities of the penitentiary system of the Russian Federation. Even though there is a trend for a reduction in the level of juvenile crimes in Russia, new approaches to their prevention are needed. The effectiveness of correctional influence directly depends on the systemic and complex character of its implementation. It is necessary to take into account the criminological characteristics of the analyzed category of convicts, which will make it possible to fully and accurately determine key trends of preventing repeat offences by juveniles and of helping their re-socialization. This work should be based on data that objectively reflect the characteristics of inmates in educational penal colonies. One of the fundamental ways of obtaining objective data is a census. The goal of this research was to carry out a census of all underage inmates in the penitentiary institutions in October 2019, to collect information on the criminological characteristics of minors, to identify the trends for their changes and to use the obtained information in the preventive work with them. The authors studied all inmates (1 214 people) in all 23 penitentiary institutions for juvenile delinquents in Russia. Research results were used to create a generalized criminological portrait of a juvenile inmate. It is a male person (91,5 %), 16-17 years of age (67,3 %) who lived in an urban area prior to conviction (58,4 %), studied in a comprehensive secondary school (72,0 %), lived in a dysfunctional or socially vulnerable family (86,6 %; single-parent family - 40,4 %), who has borderline psychiatric (48,3 %) or drug-related (16,2 %) disorders (they do not impair sanity, but influence the mechanisms of deviant behavior), who has no work experience (89,6 %), no prior convictions (67,9 %) for intentional very grave crimes (50,2 %), mostly committed without accomplices. Only 8 % claim that they respect the traditions of criminal subculture and want to be professional criminals in the future. In almost half of the cases (48,4 %), the imprisonment sentence does not exceed 3 years. The obtained results can be used to improve the practice of preventive work with juvenile delinquents, their re-socialization and the future development of criminology.