Framed within a Sociocultural Theory of Mind (SCT) in the field of Second Language Acquisition (Lantolf & Thorne, 2006), this dissertation explores the role of verbalizing in the internalization of grammatical categories through the use of Concept-based Instruction (henceforth CBI) in the second language (L2) classroom. Using Vygotsky's (1986) distinction between scientific and spontaneous or everyday concepts applied to L2 development (Negueruela, 2008), this study focuses on the teaching and potential development of the grammatical concept of aspect in the Spanish L2 classroom, and the role of verbalizing in its internalization. It is proposed that verbalizing mediates between the learners' initial understandings of the grammatical concept of aspect, the development of conscious conceptualizations, and students' written and oral production of preterite and imperfect grammatical forms. This study presents and analyzes data from one of the thirty-two adult college students enrolled in an advanced Spanish conversation course. Data is analyzed through a clinical analytic approach, which has its roots in Vygotsky's (1978) genetic method of analysis. The study was carried out over a 12-week period and collected multiple sets of developmental data, including learners' definition of the grammatical concept of aspect, written performance protocols, and verbalization data recorded during two oral interviews. The study interprets learner performance in these three complementary, and dialectically connected types of L2 conceptual data. A close analysis of this participant's data provides critical insights to understand the role of verbalizing in L2 conceptual development. Findings confirm that learners' verbalizations are key factors to ascertain L2 conceptual development, as well as a mediational tool that fosters learners' internalization of the grammatical concept of aspect. It is proposed that verbalizing notably contributes to research on L2 development. Not only does it allow the researcher to have a more comprehensive picture of L2 development, but it also helps learners develop a more sophisticated semantic understanding of the grammatical concept of aspect and fosters their ability to understand and control relevant grammatical features in L2 communication. [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.]