As schools in the United States see a rise in students receiving special services, educators must adapt to meet the learning needs of this growing population. To address this need, many schools are turning to the practice of co-teaching, or the partnership of a general educator and special educator serving the same group of students. By instituting co-teaching, many schools are attempting to provide rigorous content to students with disabilities in the least restrictive environment while also providing necessary accommodations and support. The ideal co-teaching partnership exhibits parity, or equal status, between co-teachers, and this parity can be influenced by factors like content and pedagogical knowledge, compatibility, and structural elements of the environment like scheduling and school expectations. With the introduction of the Next Generation Science Standards (NGSS) in 2013, science co-teaching teams were challenged to adapt to these new demanding standards and modify instruction accordingly to make the standards accessible for all students. This shift in standards also impacted those parity factors, and little is known about how NGSS science co-teaching teams, particularly at the secondary level, are functioning within this changed standards landscape. To understand the experience of NGSS secondary science co-teachers, the roles they assume in the classroom, and the parity and cooperation they exhibit and describe in their work, one pair of co-teachers was examined in a case study via questionnaires, video and audio recorded lessons, and teacher interviews. Specifically, this research addresses the following questions: (1) What roles do secondary science co-teachers assume in the NGSS classroom? (2) How do the demands of NGSS affect the experiences of co-teachers? and (3) How do secondary science co-teachers characterize their collaboration with one another in the NGSS classroom? [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]