This work investigates middle school teachers' mathematics knowledge for teaching (MKT) as defined by Hill (2007). Within this two-part dissertation, the level of MKT was considered as well as the role of teacher beliefs in actual specialized content knowledge (SCK) use, a specific type of mathematics knowledge for teaching vital in quality mathematics instruction. Additionally, the model of MKT knowledge was explored through confirmatory factor analysis on a large, national dataset of middle school mathematics teacher survey responses involving mathematics knowledge for teaching. SCK was found to be vital in quality mathematics instruction yet not sufficient. Teacher beliefs about the delivery of mathematics instruction ultimately acted as a filter, at times limiting SCK use, even if a teacher held high levels of SCK. The mathematics knowledge that teachers hold is highly complex; confirmatory factor analysis results indicated that we have yet to truly capture the essence of MKT, yet the importance of understanding such knowledge is clearly essential. Implications for preparing future teachers are discussed. [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.]