Infant-directed maternal speech is an important component of infants’ linguistic input. However, speech from other speakers and speech directed to others constitute a large amount of the linguistic environment. What are the properties of infant-directed speech that differentiate it from other components of infants’ speech environment? To what extent should these other aspects be considered as part of the linguistic input? This review examines the characteristics of the speech input to preverbal infants, including phonological, morphological, and syntactic characteristics, specifically how these properties might support language development. While maternal, infant-directed speech is privileged in the input, other aspects of the environment, such as adult-directed speech, may also play a role. Furthermore, the input is variable in nature, dependent on the age and linguistic development of the infant, the social context, and the interaction between the infant and speakers in the environment.