The emphasis on socio-cultural theories of learning has required the understanding of multi-dimensional, dynamic and social nature of acquiring the scientific knowledge and practices. Recent policy documents suggest a focus on formative and dynamic assessment practices that will help understand and improve the complex nature of scientific learning in classrooms. This study focuses on teachers' use of "Informal Formative Assessments (IFA)" aimed at improving students' learning and teachers' frequent recognition of students' learning process. The study was designed as an ethnographic case study of four middle school teachers and their students at a local charter school. The data of the study included (a) teachers' responses to history of teaching questionnaire (b) video and audio records of teachers' assessment practices during two different scientific projects (c) video and audio records of ethnographic interviews with teachers during their reflections on their practices, and (d) field notes taken by the researcher to understand the assessment culture of the school. The analytical tools from sociolinguistics (e.g., transcripts and event maps) were prepared and discourse analysis based in an ethnographic perspective was used to analyze the data. Moreover, Cultural-Historical Activity Theory (CHAT) was also introduced as an alternative data analysis framework for understanding the role of division of labor among the elements of the community on the challenges and the outcomes of IFA practices. The findings from the analysis of the classroom discourse showed three different types of IFA cycles: connected, non-connected, and repeating. The analysis of the teachers' reflections showed that the effectiveness of these cycles did not only depend on whether the cycles were connected, but also on other variables such as the phase of the lessons and student's identities and abilities. Teachers' reflections during researcher-teacher meetings on the concept and the aims of IFA improved through the use of academic literature on IFA and video-cases of their own practice. Teachers also reflected on the challenges for effective implementations of IFA and they emphasized challenges due to the division of labor among the classroom participants and the open nature of scientific knowledge. Through participation in the study, the teachers helped develop an IFA model for middle school science classrooms designed to capture the complex nature of teacher-student interaction. This model can be used for further analysis of IFA activity and professional development activities focused on assessment practices. [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/en-US/products/dissertations/individuals.shtml.]