The work presented in the current thesis was aimed at identifying the exact locus of the age of acquisition (AoA) effect within the systems responsible for word and picture processing. Chapter One reviews some of the current influential models of word and picture production and discusses the effects that AoA (and frequency) have upon these processes. Current theories of AoA are also discussed. Chapters Two and Three assess the locus of the AoA effect in the word naming task. The results of these experiments lead to the conclusion that AoA (and frequency) exert their effects in the connections between orthography and phonology in single word naming. Chapter Four then tested the alternative claim that AoA affects the level of phonological output processing by investigating the AoA effect in a phonological segmentation task and by relating the size of the AoA effect in this task and in a word naming task to individual differences in phonological skill. The results of this comparison demonstrate that AoA is unrelated to explicit phonological processing. Chapter Six then investigated the effect of AoA (and other variables) in the picture naming task by relating aphasic patient's level of impairment to the variables that affect their picture naming performance. The results of this study suggest that AoA influences the strength of the connections between semantics and phonology in picture naming. The present thesis concludes that AoA influences the strength of the connections between input (orthography and semantics) and phonological output. The final Chapter discusses the implications of the present results for current theories of AoA and for models of word and picture production.