A series of studies investigated whether infants can detect cues in ongoing speech that could help them delineate those segments that correspond to grammatical units like phrases and words. The methodology of the study involved asking whether infants show a preference between speech samples in which pauses have been inserted coincident with the boundaries of the unit in question versus those in which pauses have been inserted at a location internal to the unit. Findings, overall, indicate that by the age of 4 and one-half months infants show a sensitivity to prosodic cues in speech that are correlated with speech segments that span clausal units in language, and this sensitivity obtains whether the language is English or whether it is an unfamiliar language, Polish. By about 6 months, this sensitivity has narrowed to English. But 6-month-olds do not appear to show a sensitivity to cues that are correlated with speech segments that correspond to finer linguistic units. Not until 9 months of age can infants detect cues that can mark segments corresponding to the clause-internal strucure formed by the major phrase constituents. Not until 11 months of age is there apparent sensitivity to cues that correlate with the packaging of syllable strings into unfamiliar words. Possible interpretations of the findings are discussed. (RH)