This dissertation investigates optimal conditions for form-focused instruction (FFI) by considering effects of internal (i.e., timing and types of FFI) and external (i.e., complexity and familiarity) variables of FFI when it is offered within a primarily meaning-focused context of adult second language (L2) learning. Ninety-two Korean-speaking learners of English as a foreign language (EFL) participated in the study and were randomly assigned to one of five experimental groups or one control group. The experimental participants were instructed in one simple and one complex targets through (1) deductive FFI followed by meaning-focused instruction (MFI) (DM); (2) deductive FFI preceded by MFI (MD); (3) inductive FFI followed by MFI (IM); (4) inductive FFI preceded by MFI (MI); or (5) MFI-only. No instruction was provided for the controls. Learning was measured by a grammaticality judgment task (GJT), and knowledge types were examined by subjective measures of awareness (i.e., confidence ratings and source attributions), an oral elicited imitation task (OEIT), and a metalinguistic knowledge test (MKT). The results indicate that the combinations of FFI and MFI result in more robust learning effects than the exclusive use of MFI. This was demonstrated by the mixed-method learners' GJT accuracy rates, which were significantly higher than the controls' immediately and over time. As for the target complexity and item familiarity, the analyses show that only the mixed-method learners significantly outperformed the controls when the target was more complex and the targets were embedded in unfamiliar syntactic patterns. As for the types of knowledge, the analyses found confirmative evidence that FFI positively influences the development not only of explicit knowledge but also of implicit knowledge in the long term. The MFI-only group demonstrated evidence of explicit knowledge but not implicit knowledge. The findings suggest that adult L2 learners may benefit considerably more from the combination of FFI and MFI, particularly for complex rule learning and development of rule transfer ability. Moreover, the findings indicate that possession of explicit knowledge may not hinder development of implicit knowledge. Rather, explicit L2 knowledge may offer a foundation for parallel development of implicit knowledge. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]