Infant mental health problems include difficulties to regulate emotions or attention, crying, sleeping or feeding problems as well as aggressive behavior. Early identifications of these problems help to change developmental trajectories and improve developmental outcomes. Psychiatric assessment and classification have to take into account the rapid processes of development as well as the inseparable linkage between symptoms of the infant, psychosocial risks in the family environment, and parent–child relations. The proposed DSM-5 classification system presents a systematic description of mental health disorders which are relevant for infant psychiatry. However, the proposal has provided rather limited attention to developmental differences and parent–infant relations. Therefore, additional classification systems, like the Zero-to-Three (DC: 0–3R), are strongly recommended. In terms of assessment and in accordance with the guidelines of the American Academy of Child and Adolescent Psychiatry, infant psychiatrists have to consider the close relation between somatic and mental health and the interplay between behaviors of the caregiver and the infant. Therefore, the assessment has to be multidisciplinary and relationship based. A standard assessment in infancy includes a clinical interview, behavior observations, caregiver questionnaires, and a pediatric screening. All assessments should pay attention to motor, cognitive, language, and social-emotional development. Because infant development is embedded in the family context, socio-economic factors, parents’ mental problems, including drug abuse, domestic violence, and trauma history should be assessed. The treatment has to be oriented toward symptoms and development and has to address underlying medical conditions. The focus should be on parent–child interactions. Evidence-based interventions are based on attachment theory, use social-learning perspectives, and behavioral approaches.