This study explored K-12 teachers' understanding and implementation of the Next Generation Science Standards (NGSS) during and after participation in a professional development (PD) program that included the development of science teachers' conceptual understanding of science. We add to the literature with our focus on a multi-year PD program emphasizing the vertical progression of concept development from kindergarten to 12th grade, rich engagement in science and engineering practices and crosscutting concepts, deep understanding of NGSS, and collaborative discussion to develop research-based pedagogical strategies to teach the three dimensions. In particular, we focus on foregrounding/backgrounding dimensions throughout a science unit to simplify instruction. Through an exploratory qualitative approach, we sought to answer the following research question: "During a three-year professional development program, how do K-12 teachers develop an actionable understanding of the intertwining three dimensions of the Next Generation Science Standards?" Teachers participating in all three years of the project were involved in school-based focus group interviews to elicit their understanding and implementation of the NGSS, especially regarding the interweaving nature of the three dimensions of the NGSS. Findings suggested that although the standards are complex, it is critical to be explicit about the three dimensions and intentional about planning for instruction. Collaboration in vertical teams and deep reflection on content and pedagogy were essential elements of the PD program. This study offers insight into the time it may take for individuals to substantially shift their daily teaching practices, underscoring the complexity of the standards and teaching shift we are asking of our teachers. Thus, foregrounding/ backgrounding the dimensions throughout a unit may support teachers' actionable understanding of NGSS.