Schmidt (1990) claimed that consciousness, in the sense of awareness of the form of input at the level of “noticing”, is necessary to subsequent second language acquisition (SLA). This claim runs counter to Krashen's (1981) dualsystem hypothesis that SLA largely results from an unconscious “acquisition” system, the contribution of the conscious “learning” system to SLA being limited and peripheral. Important to a theory of SLA that allows a central role to the act of noticing is a specification of the nature of the attentional mechanisms involved, and of their relationship to current models of the organization of memory. With this in mind the present paper reviews current research into the nature of attention and memory and proposes a model of the relationship between them during SLA that, it is argued, is complementary to Schmidt's noticing hypothesis and oppositional to the dual-system hypothesis of Krashen. In light of this model, I argue that differential performance on implicit and explicit learning and memory experiments is caused by differences in the consciously regulated processing demands of training tasks and not by the activation of consciously and unconsciously accessed systems. I also argue that the attentional demands of pedagogical tasks and individual differences in memory and attentional capacity both affect the extent of noticing, thereby directly influencing SLA.