During their first months of life, infants’ language input consists primarily of infant-directed speech (IDS), a speech register that is structurally, prosodically, and acoustically different from adult-directed speech (ADS). IDS is a dynamic register: infants’ preferences and responses to IDS evolve as they develop more mature linguistic abilities, and caregivers, in turn, adjust the quantity and quality of IDS to their infants’ listening preferences and developmental needs. Based on this, it has been proposed that IDS provides infants with an optimal speech signal to facilitate speech processing and encoding at the early stages of language development. In this talk, I will present recent studies investigating this proposal by measuring neurophysiological responses to IDS and ADS in monolingual and bilingual infants. Our findings indicate that IDS plays an important role in facilitating neural encoding of speech, but infants’ neural responses to IDS are modulated by their age, familiarity with the speech stimuli, and the complexity of the speech processing task.