Infant- or child-directed speech (CDS) defines the code used to communicate with infants or children, which differs from standard adult speech in prosody, expressions, diction and word repetition etc. A selective literature search in PubMed was carried out for the purposes of this systematic review. Due to its specific advantages, child-directed speech facilitates the extraction and representation of relevant, meaningful sections from the continuous speech signal. Different speech communities use different variants of CDS. CDS is not only seen to be used by adults, but also by children communicating with younger children. However, there are speech communities that do not use CDS. Taking into consideration findings previously described in the literature, CDS appears to positively support language acquisition in children, but does not represent a necessary prerequisite. However, there are no findings in the literature to indicate that the linguistically reduced CDS hinders early language acquisition.