The aim of the article is to discuss the standards of clinical diagnosis in children and adolescents in the context of theoretical models and psychological practice. Contemporary clinical practice standards refer to evidence-based practice (including evidence-based psychological practice, EBPP). A correct interdisciplinary diagnosis, as in the global standards, is primarily required due to an observed increase in the prevalence of emotional problems, behavioural difficulties and mental disorders among younger and younger children. The clinical psychological diagnostic process plays a key role in this respect. It is a process that takes into account both the individual and developmental context as well as the biopsychosocial nature of difficulties experienced by the child. Inference based on the interview, observation and results of standardised psychological tools, referring to clinical research and scientific theories, is an essential part of this process. The paper discusses in detail the worldwide standards of a clinical psychological diagnostic process and the specificity of clinical diagnosis in children and adolescents. At the same time, guidelines for clinical psychological diagnosis in children and adolescents have been proposed in relation to the Polish realities of clinical practice. The authors prepared and shared a list of psychological tests for use in clinical assessment of children and adolescents. The list is divided based on the studied development period (early childhood, pre-school age, school age, adolescence) and the specificity of measuring psychological variables (diagnosis of development, intelligence, cognitive abilities, emotional and social competence and personality, language development, family context, symptoms of mental disorders). The proposed psychological test list corresponds to the contemporary approach to clinical diagnosis, taking into account both the psychological profile and the socio-cultural context of the examined person’s functioning (psychosocial diagnosis). The evidence-based practice and the proposed list of psychological tools assume that good clinical practice should be based not only on the clinician’s experience, but above all on verified, reliable data and available scientific research.