There has been a declining interest in science by adolescents as they mature in the American school system (Anderman & Young, 1994). Inquiry-based teaching is a supported practice with the aim of shifting science teaching to better engage students. This study offers a qualitative analysis of teachers' perceptions and implementation of inquiry-based teaching in secondary classrooms by examining perceptions, co-planning, assessment selection, and classroom instruction. By using survey data, applying the Science Teacher Inquiry Rubric (STIR) as an observation tool, and collecting qualitative data from teacher planning documents and assessments, the present study fills a critical gap in inquiry-based teaching scholarship. The results show that teachers think that inquiry involves student-centered activities and curricular agency, that the teacher's role is that of a facilitator, and that inquiry yields a practice or product. Teachers use online resources and previously outlined units and focus their attention on student outcome goals during co-planning sessions. The STIR data revealed that the student assessments and classroom experiences were teacher-centered. [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.]