The present paper reviews recent studies on the early segmentation of word forms from fluent speech. After having exposed the importance of this issue from a developmental point of view, we summarize studies conducted on this issue with American English-learning infants. These studies show that segmentation abilities emerge around 8 months, develop during the following months, and rely on infants' processing of various word boundary cues the relative weight of which changes across development. Given that these studies show that infants mostly use cues that are specific to the language they are acquiring, we underline that the development of these abilities should vary cross-linguistically, and raise the issue of the developmental origin of segmentation abilities. We then offer one solution to both the crosslinguistic differences (also observed in adulthood) and bootstrapping issues in the form of the early rhythmic segmentation hypothesis. This hypothesis states that infants rely on the underlying rhythmic unit of their native language at the onset of segmentation abilities: the trochaic unit for stress-based languages, the syllable for syllable-based languages. After the presentation of various elements on which this hypothesis relies, we present recent data on French infants offering a first validation of this proposal.What does it mean, from a developmental point of view, to learn a lexicon? A word corresponds to the specific pairing between the mental representation of a sound pattern (word form) and an abstract representation (concept) of an object or event in the world that constitutes the meaning associated to that word form. The building of a lexicon will then rely on the development of three sets of abilities: the ability to elaborate and store word 38 Karima Mersad, Louise Goyet & Thierry Nazzi forms, the ability to build concepts for the objects and events in the world, finally the ability to appropriately link word forms and concepts. Note that the elaboration of word forms and of concepts starts before the onset of lexical acquisition per se around the ages of 10 to 12 months. It is thus likely that infants in their first year of life constitute a store of word forms and concepts that are later paired to make words. In the second year of life, all these acquisitions could happen simultaneously (see Nazzi & Bertoncini, 2003, for a discussion of possible changes in word acquisition around 18 months of age).The acquisition of the lexicon by infants and young children has been a research topic for decades. In what follows, we review studies focusing on a specific ability involved in word learning, namely the ability to extract the sound pattern of words from fluent speech (henceforward, word form segmentation). Word form segmentation constitutes a crucial step in speech processing, which allows infants as well as adults to determine the sequence of lexical units that constitute the utterances they hear. The ability to extract word forms from spoken speech might play a critical role for th...