According to Slobin (1993, 1996), recognizing the meanings of linguistic concepts is exceptionally difficult for adult second language learners, because they are often non-compositional elements of semantic structure, i.e., they are not perceivable as part of the linguistic environment. Carroll & von Stutterheim (2003) argue further that such elements may be unlearnable in adult SLA. The dissertation investigates this presumed learning difficulty by examining the potentially facilitative effects of concept-based instruction and/or input flood over extensive reading alone for the acquisition of the meanings of German inseparable prefixes ('be'- and 'er'-) and prefixed words. The prefixes' prototypical meaningfulness was analyzed through the framework of cognitive linguistics (Lakoff, 1987; Langacker, 1991), and a pedagogical explanation was developed that utilized an image schema diagram and an explanation of the metaphorical and metonymic meaning shifts that the prefixes contribute to prefixed words. The research addressed the following questions: (1) can advanced, adult second language learners develop an understanding of conceptual constructs containing multiple linguistic concepts that are encoded by different grammatical features in their first and second languages; (2) can they differentiate the encoding domains of different members of the same grammatical feature; (3) do instructional interventions that focus the learners' attention on the systematic meaningfulness of the prefixes lead to greater gains in vocabulary knowledge of prefixed words than a condition in which learners read for meaning; and (4) is an explicit/deductive pedagogical intervention or an explicit/inductive intervention more effective for maximizing prefix and word learning ? Findings include: (1) learners who participated in concept-based instruction followed by enhanced input benefited significantly in their understanding of the meanings of the prefixes; (2) learners from all groups showed gains in prefixed word knowledge over time; however, instructed learners (explicit/deductive learning condition) outperformed learners with access to enhanced input through computerized input flood (explicit/inductive learning condition) and learners who read texts for meaning. Additionally, learners provided evidence that they tap a variety of knowledge sources to produce sentences and provide definitions for prefixed words, including both L1 and L2 semantic knowledge, episodic memory, contextualized exemplars, and knowledge of the prefixes' and root words' meanings. (Katherine Sprang)