Mental retardation (MR) has a prevalence of about 2%, and developmental speech disorders with unknown cause about 7%. Very often, developmental delay represents an early warning sign of later developmental problems, such as mental retardation and other developmental disorders (DD). Prognosis depends on many factors, most notably the limiting factor provided by the severity of the underlying brain dysfunction and its consequences for communication, cognition and other behaviour. Chromosomal aberrations are the major cause of MR and also play a role in mild forms of DD and speech acquisition delay. ENT specialists are confronted with children with DD or language acquisition disorders. If the suspicion of hearing impairment as the cause of problems in daily life is not confirmed in a patient, ENT specialists should also consider genetic causes in the differential diagnosis.