Mental Health Disorders in Infants, Toddlers, and Preschool Children: Basic Aspects of Their Assessment and Treatment as well as Relationships, Crying, and Sleep Disorders. A Clinical Guideline for Assessment, Counseling, and Treatment (Part 1) Abstract: Objective: Mental health disorders are just as common among young children aged 0 to 5;11 years as in older children and adolescents. These disorders are heterogeneous and can be incapacitating. They vary depending on the developmental stage of the child but can be diagnosed and treated effectively. The aim of revising these interdisciplinary guidelines is to present the current state of knowledge on the assessment and therapy of the most important disorders. We formulate consensus-based, practical recommendations. The first part focuses on the basic principles of assessment, counseling, and treatment. In addition, it covers relationships, crying, and sleep disorders. Methods: The Guideline Commission consisted of 18 professional associations. The guideline was based on current literature searches and several online consensus conferences, following standard procedures. Results: A total of 16 different groups of mental health disorders could be identified among young children. Relationship disorders and the relational context play an important role in this age group. In addition to standard assessment procedures, specific diagnostic approaches can be helpful if indicated. The diagnoses should be based on the DC: 0 - 5 and/or other classification systems such as the ICD-10. Counseling should be offered in every case. If indicated, specific psychotherapeutic approaches for each disorder are recommended, including the involvement of parents and other caregivers. Medication is usually not indicated. Specifically, excessive crying and sleep disorders, which predominantly affect infants and toddlers, can be treated effectively. Conclusions: The recommendations of this guideline were adopted with a high consensus. The recommendations regarding assessment, counseling, and treatment can be implemented in different clinical contexts and by various professional groups. More research is required for some disorders.