Infant mental health is a growing research area, but findings have not generally translated into new service developments. Recognising mental health problems in young infants is relevant for clinicians in all mental health specialties, but it can be a particular challenge to make diagnoses in very young children. Mental health classification systems are fraught with the difficulties of standardising diagnoses for infants, while trying to provide a clinically useful and relevant framework. The diagnostic classification DC:0–3 appears to have strengths, for example, a clear space to consider relationship disorders, and therefore encouraging a broad assessment of the child and family. More information is beginning to gather regarding infant mental health services around the world and assessment of this patient group in clinical practice. This commentary aims to help inform clinicians about this developing field.