This study evaluated the influence of general and mathematics-specific classroom instructional quality on the mathematical development of a sample of Head Start preschool children. It also examined whether children's age, pre-test mathematical knowledge, and English language learner status influenced the quality of their curricular experiences and subsequent mathematical development. Findings indicate that mathematics-specific measures of quality, specifically curriculum intensity, significantly influenced children's mathematical gains, while broader measures of instructional quality, as measured by general cognitive support, did not significantly predict mathematical development. The quality of mathematical instruction and children's mathematical development were also found to be unrelated to general instructional quality or overall classroom quality. Results also indicate that children who entered preschool with more mathematical knowledge experienced more intense curricular experiences. Finally, the influence of ELL status was significant, with Asian-American ELL children experiencing greater curriculum intensity and making greater mathematical gains from pre-test to post-test than both their ELL and predominantly English speaking peers. This study reaffirms the importance of curricular intensity in preschool mathematics instruction. It also provides preliminary evidence suggesting that teachers' mathematical instruction may be a distinct early childhood content area, which does not necessarily relate to the overall quality of the classroom or to teachers' general instructional quality. Implications include more planned mathematical instruction in early education settings, the inclusion of formative assessments in early childhood curriculum packages, improvements to teacher training programs in preschool mathematics, and ongoing professional development to ensure that early childhood educators continually make improvements to the mathematical quality of their instruction. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]