The psychological assessment of offenders throughout the different stages in the juvenile justice system is essential. It ensures the adequacy of the legal and educational measures to be applied in the process. This paper reviews the main tests of psychological assessment available in Spanish, suitable for use by psychology professionals who work with young offenders in the juvenile justice services in Spanish-speaking countries. We classify these tools into three groups: a) personological, i.e. generic tools, suitable for any professional context in psychology, b) clinical, i.e. tools whose initial use has been limited to working with adolescents with mental health needs, and c) forensic, tools that have been specially developed for use in the juvenile justice population. This last group is described in the second part of this article (which appears in this same issue). The most important instruments of proven utility are presented and reviewed for each group.