<p>The question of whether second languages (L2s) are best learned implicitly or explicitly has been a topic of much empirical discourse, with the majority of studies pointing to the benefits of explicit instruction when learning L2 grammar rules. However, given the focus on grammar, it is unclear how generalizable these findings are to other linguistic domains, such as L2 speech and L2 vocabulary. The previous focus on laboratory-based settings, and the language bias in the literature, also make it unclear how ecologically valid and applicable these findings are to the real world. To address these macro research questions, two experiments were carried out on English-speaking L2 learners of German.</p> <p>Experiment I (ExI) investigated the effects of implicit and explicit learning on the acquisition of Final Obstruent Devoicing and Dorsal Fricative Assimilation. The effect of the two learning conditions on L2 perception was also measured using a perceptual discrimination task and a perceptual identification task. Experiment II (ExII) investigated the effects of explicit historical instruction on the learning of English-German cognates, which were compared to the effects of a non-explicit learning condition. To examine whether declarative knowledge of relevant historical changes can aid in vocabulary learning, an explicit condition received instruction on the Second Germanic Sound Shift, Ingvæonic Palatalization, and relevant historical semantic changes. Both experiments followed a pre-/post-/delayed-post-test design.</p> <p>Results indicate that the two explicit conditions significantly outperformed the non-explicit conditions, suggesting that explicit learning and explicit instruction can be beneficial when learning L2 speech and L2 vocabulary. In ExI, acoustic analyses of learner speech samples indicate that the explicit condition was more successful in the learning of the two phonological rules. In ExII, the explicit condition was more successful in the identification and learning of cognates, suggesting that knowledge of language history, and instruction on applied historical linguistics, can be beneficial when learning a language that is historically related to a language that learners already speak. The results from this dissertation are discussed in the context of implicit and explicit learning and instruction, the role of attention, and the role of declarative knowledge, with concluding remarks pointing to the importance of metacognitive and metalinguistic awareness in adult or university-level language courses.</p>