Dual language and immersion (DLI) instruction at the secondary level is a particular challenge, as teachers are tasked with teaching abstract, cognitively complex content while supporting students in continuing to develop their partner language proficiency. This content and language integration requires a unique skill set and knowledge base, but secondary DLI teachers rarely have DLI-specific teacher preparation. Moreover, secondary DLI teachers face an additional hurdle in that almost no pedagogical materials exist that are designed specifically to support the integration of content and language. This dissertation study sought to address both of these difficulties by (re)designing two secondary Spanish DLI teachers' content-based pedagogical materials to include language-focused modifications with the ultimate goals of developing the teachers' knowledge for content and language integration and fostering integrated instruction in practice. For this study, I collaborated closely with two teachers over the course of the 2020 academic year, and the teachers' participation in the study served as a professional development experience. Drawing on design-based research (McKenney & Reeves, 2012), we completed iterative cycles of collaborative materials analysis, (re)design of materials, instruction with modified materials, and collaborative reflection. Data sources included formal interviews, informal conversations, collaborative planning and reflection sessions, and classroom observations. This study included both research on the intervention, as I examined how the teachers used the (re)designed materials, and research through the intervention, as I explored how participating in the study shaped the teachers' conceptualizations and enactments of content and language integration. Findings suggest that pedagogical materials have the potential for supporting secondary DLI teachers in integrating content and language in their instruction. However, well-designed materials must include features that educate teachers about discipline, language, and literacy connections as well as pedagogical practices for integration. Furthermore, this study demonstrates that secondary teachers' understanding of content and language integration is grounded in their conceptualization of language itself. Therefore, secondary DLI professional development programs should include a focus on language-content relationships. This dissertation concludes with theoretical contributions about DLI teachers knowledge, design principles for future research on materials as a resource for content and language integration, and implications for DLI teachers development. [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.]