It has been suggested that much of the information we acquire from our external environment involves processes that do not require conscious awareness (e.g. Reber, 1989; Reber and Winter, 1994). Such knowledge acquisition has been termed implicit learning and this has been put forward as a fundamental process in allowing learning of complex information (e.g. Reber, 1992; Schmidke and Heuer, 1997). It has been proposed that acquisition of the underlying rule structure of stimulus events provides an indication of such a process as being fundamental and general. In contrast, learning bound to more peripheral processes should only be shown when subjects learn, for example, surface features of stimuli or a sequence of motor responses, but not the underlying rules (e.g. Perruchet and Pacteau, 1990; Seger, 1998). The research in this thesis investigates systematically whether implicit learning of sound stimuli behaves any differently to such learning of visual stimuli. This expands the empirical scope of previous studies in the implicit learning field and allows assessment of such processes as fundamental and general. Chapter 1 provides a background to implicit learning in general and introduces the different concepts involved. Chapters 2 to 4 investigated the generality of findings from visual implicit learning studies in the auditory domain. In particular, they studied the role of rule abstraction in sequence learning (Nissen and Bullemer, 1987) and invariant learning tasks (McGeorge and Burton, 1990). Findings from the sequence learning experiments in Chapters 2 and 3 suggest that subjects were unable to abstract the underlying rule structure of stimuli, as would have been evident from learning of the auditory sequences employed by listening alone. Instead, subjects were only able to learn the relevant associations between their actions (keypress responses) and a set of stimuli. These findings add to evidence from visual implicit learning studies that found peripheral processes involved in such learning. Findings from the invariant learning experiments in Chapter 4 show what types of auditory invariant features subjects can and cannot learn. This identified for the first time the exact information, or rule, that subjects acquire in such a task in an auditory context. Additionally, it provides some evidence that explicit processes may have been involved. Overall, the findings from the experiments in this thesis put into question that implicit learning is a fundamental process, which involves implicit rule abstraction.