Elementary school teachers in the United States are typically provided curricular materials by their school districts. However, little is known about how teachers may replace or supplement these provided materials with outside materials—materials found in their own curricular libraries or from external sources. This dissertation proposes a research framework on teachers’ use of outside materials that leads to the following research questions: What motivates teachers to consider using outside curricular materials? What approaches do they use to discover outside materials? What criteria do they use to evaluate outside materials? What steps do they take to prepare or adapt outside materials? And what external factors relate to teachers’ decisions to use outside materials? These questions were addressed through an online survey of 98 elementary teachers across two districts in Western Washington state, interviews with nine survey respondents who frequently considered using outside materials, and in-depth case studies of the decisions of three interviewees. The survey revealed that a large majority of teachers considered using outside curricular materials at least once in the 2015-2016 school year, that many of them did so frequently, and that their reasons for considering outside materials were linked to areas of dissatisfaction with the provided materials. A multiple linear regression found that higher teacher-reported feelings of curricular autonomy and more years of experience with their provided curriculum predicted higher frequency of considering outside materials. The survey also revealed that teachers used a wide variety of approaches to discover outside materials, including both online and offline methods. Interviews provided detail to these findings, revealing that teachers’ evaluation criteria were aligned with their initial motivation to consider outside materials, and that teachers tended to engage in minimal preparation of outside materials. Case studies of three focal teachers were used to illustrate different motivations for considering outside materials and different approaches for discovery, as well as to elaborate and refine the research framework that guided this investigation. This dissertation adds to literature showing that teachers actively participate with curricular materials in order to reach their professional goals, and makes a strong case for including attention to outside materials in curriculum development, district-level decisions, and future research.