It has often been observed both anecdotally and experimentally that English /r/ and /l/ are hard to acquire for children (Ingram 1989, Gildersleeve-Neumann 2000) as well as second language learners. This talk reports on ongoing work at the UBC Interdisciplinary Speech Research Laboratory (ISRL) investigating the possibility that this difficulty is due to specific issues in the development of tongue motor control. In the few instrumental studies of across-the-board substitutions in children’s speech, these have been found to have a motor rather than a phonological basis (a.k.a. ‘covert contrast’; Gibbon 1990, Scobbie & al. 2000). Various proposals will be evaluated, such as multiple articulatory-acoustic mappings and articulatory complexity in terms of number and timing of gestures. However, the liquids are shown to be unique in English in that they comprise multiple lingual gestures, leading to the proposal that differentiating within rather than across articulators is more complex. This proposal has led to a number of studies, including an articulatory (ultrasound/EPG) study of severely hearing-impaired high school students who communicate primarily with speech (rather than signing). It was predicted, and found, that as they lacked sufficient acoustic input to force lingual differentiation, these speakers would have markedly under-differentiated tongue control. Likewise, a cross-linguistic ultrasound study supports the notion that liquids are easier to acquire in languages not requiring tongue differentiation for these segments. Gibbon, F. 1990. Lingual activity in two speech-disordered children’s attempts to produce velar and alveolar stop consonants: Evidence from electropalatographic (EPG) data. British Journal of Communication, 25, 329-340. Gildersleeve-Neumann, C. E., B. L. Davis, and P. F. MacNeilage. 2000. Contingencies governing the production of fricatives, affricates, and liquids in babbling. Applied Psycholinguistics 21, 341-363. Ingram, D. 1989. First Language Acquisition. Cambridge: CUP. Scobbie, J. M., F. Gibbon, W. J. Hardcastle, and P. Fletcher. 2000. Covert contrast as a stage in the acquisition of phonetics and phonology. In Broe and Pierrehumbert (eds.) Papers in Laboratory Phonology V: Acquisition and the Lexicon. Cambridge: CUP. 194-207.