AbstractThe research reported in this study focuses on the design and evaluation of learning environments that support the teaching and learning of argumentation in a scientific context. The research took place over 2 years, between 1999 and 2001, in junior high schools in the greater London area. The research was conducted in two phases. In phase 1, working with a group of 12 science teachers, the main emphasis was to develop sets of materials and strategies to support argumentation in the classroom, and to support and assess teachers' development with teaching argumentation. Data were collected by video‐ and audio‐recording the teachers' attempts to implement these lessons at the beginning and end of the year. During this phase, analytical tools for evaluating the quality of argumentation were developed based on Toulmin's argument pattern. Analysis of the data shows that there was significant development in the majority of teachers use of argumentation across the year. Results indicate that the pattern of use of argumentation is teacher‐specific, as is the nature of the change. In phase 2 of the project, the focus of this paper, teachers taught the experimental groups a minimum of nine lessons which involved socioscientific or scientific argumentation. In addition, these teachers taught similar lessons to a comparison group at the beginning and end of the year. The purpose of this research was to assess the progression in student capabilities with argumentation. For this purpose, data were collected from 33 lessons by video‐taping two groups of four students in each class engaging in argumentation. Using a framework for evaluating the nature of the discourse and its quality developed from Toulmin's argument pattern, the findings show that there was improvement in the quality of students' argumentation. This research presents new methodological developments for work in this field. © 2004 Wiley Periodicals, Inc. J Res Sci Teach 41: 994–1020, 2004