This study examined the second language (L2) development of adult learners of Spanish at three levels of proficiency during and after a semester of instruction. A fundamental goal was to identify cognitive and psychosocial individual differences (IDs) that can explain between-learner variation over time in order to expand our understanding of the role learner IDs play in L2 learning and retention. This study also advances a line of research aiming to improve the validity and reliability of tests designed to measure knowledge "of" (implicit) and "about" (explicit) L2 grammar. Eighty-seven learners in their 2nd, 4th, and 6th semester of university Spanish instruction completed four tests of cognitive ability measuring L2 aptitude, working memory capacity (WMC), phonological short-term memory (PSTM) and processing speed as well as two Likert-type questionnaires measuring different facets of L2 motivation. Implicit and explicit knowledge of ten linguistic structures in Spanish were measured through an elicited oral imitation task and untimed grammaticality judgment task used in previous research with several methodological improvements to strengthen their construct validity. A multi-faceted statistical approach was taken to answer research questions employing both traditional and innovative hierarchical linear modeling techniques. Results suggest that 2.5 months of classroom instruction significantly enhanced performance on both measures of L2 knowledge for all proficiency groups with continued gains one month later. Higher proficiency groups significantly outperformed lower proficiency groups at each point in time and there was evidence of faster and more efficient use of implicit linguistic knowledge with increasing proficiency. However, exploratory and confirmatory factor analyses revealed that the validity of a two-factor model varied according to learners' proficiency level, suggesting that implicit and explicit knowledge in instructed learners is best viewed as a continuum. For lower proficiency groups, several strong relationships were found between cognitive capacity, L2 motivation, and performance on measures of implicit and explicit knowledge; however, learner IDs did not explain performance variance for the advanced group. Relationships among ID constructs also highlight their likely interaction and dynamic influence on L2 development over time. Findings are discussed in terms of their implications for second language acquisition (SLA) theory, assessment, and pedagogy. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]