Language development is frequently characterized as a process where learning proceeds implicitly, that is, incidentally and in absence of awareness of what was learned. This article reports the results of two experiments that investigated whether second language acquisition can also result in implicit knowledge. Adult learners were trained on an artificial language under incidental learning conditions and then tested by means of grammaticality judgments and subjective measures of awareness. The results indicate that incidental exposure to second language syntax can result in unconscious knowledge, which suggests that at least some of the learning in this experiment was implicit. At the same time, however, it was also found that conscious (but unverbalizable) knowledge was clearly linked to improved performance in the grammaticality judgment task.