The purpose of this study is to provide a theoretical explanation for the effectiveness of Written Corrective Feedback (WCF) in increasing second-language (L2) students' grammatical accuracy. WCF is examined via Skill Acquisition Theory (SAT) in order to account for uneven patterns of its effectiveness. As the study demonstrates, WCF is effective at increasing accuracy for some grammatical categories, but not for others. This SAT analysis also clarifies the seemingly contradictory findings in the literature regarding WCF's overall effectiveness. Under SAT, for an instructional strategy for grammar to be effective, it must: (1) impart adequate and clear declarative knowledge of grammatical constructions and (2) provide a practice procedure whereby L2 students can proceduralize and eventually automatize grammatical knowledge. WCF that can accomplish these two goals is effective at increasing L2 students' grammatical accuracy over time, whereas WCF that cannot is ineffective even when used over a substantial span of time. The study has two major components. First, a quantitative study examines the rate of L2-English students' successful corrections on final drafts in response to WCF provided by the teacher in their first drafts (i.e., evidence of uptake). WCF in this study is a popularly used variety of indirect WCF. Descriptive statistics demonstrate that students corrected errors designated as "simple" (which involved a binary choice), but they either ignored feedback or made incorrect revisions to errors designated as "complex" (which involved a non-binary choice). Statistical analysis of the data demonstrates a statistically significant difference for WCF effectiveness on simple versus complex grammatical categories. The second phase of the study examines WCF effectiveness for improving L2-Spanish students' grammatical accuracy (i.e., evidence of proceduralization) on new written works over time. The same type of indirect WCF was used as in the English phase of the study. The Spanish study was important in order to confirm the binary versus non-binary contrast by looking especially at idiosyncratic and non-idiosyncratic binary contrasts in addition to non-binary contrasts. Using a one-way repeated measures analysis of variance (repeated ANOVA), it was found that the grammatical accuracy improved for experimental group participants compared to control group participants for non-idiosyncratic binary contrasts, but not idiosyncratic binary contrasts. As with the previous study, WCF was also not successful for non-binary (i.e. > binary) contrasts. Thus, constructions designated as "simple" (i.e., treatable via this type of WCF) are only those involving non-idiosyncratic binary contrasts. Constructions designated as "complex" (i.e., not treatable via this type of WCF) must be expanded to include not only non-binary contrasts but also grammatical paradigms with idiosyncratic binary contrasts. This combination of quantitative and qualitative analyses of multiple sources of data from two different L2-language learning groups provides strong empirical evidence in support of SAT analyses of L2 instructional strategies like WCF. More precisely, these findings elucidate the reasons for WCF's variable effectiveness, providing a predictive utility for when L2 instructors can expect WCF to work or not. Specifically, only constructions with two possible choices with a non-idiosyncratic distribution improve over time through the use of WCF, such as the singular-plural contrast of subject-verb agreement in English. However, for a binary choice for which the two options have an idiosyncratic distribution, WCF is not effective for improving the whole grammatical category but can improve usage item-by-item, such as the Spanish masculine/feminine contrast for individual words. Lastly, constructions with more than two possible choices (i.e., non-binary) are also not responsive to WCF such as the three-way article choice in English of definite, indefinite, or null article. These findings demonstrate that WCF is effective at imparting sufficient declarative knowledge of whole grammatical paradigms only for binary constructions. Also, findings indicate that WCF could be effective at imparting knowledge of binary, idiosyncratic constructions, but only one item at a time. These findings are important to the study of WCF because they provide a detailed, linguistic explanation of how WCF actually works instead of simply analyzing its overall effectiveness and declaring it either to be effective or to be ineffective on the whole, which has so often been done in the literature on WCF. Under the SAT analysis used herein, "effectiveness" is defined and quantified according to its ability to impart declarative knowledge. This approach explains WCF's variable effectiveness, clarifies apparently contradictory empirical findings in the WCF research, and provides an objective measure of grammatical complexity that can be utilized in examinations of other types of corrective feedback and other L2-teaching strategies. [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. 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