Background: Density is difficult to learn and teach in middle schools. This study, hypothesizing that the density concept develops as part of a conceptual system, used a conceptual change approach to teaching density. The approach emphasized the use of multiple strategies to teach the density concept and the associated concepts in the conceptual system. Purpose: This study assessed post-instructional understanding of different aspects of density in a sample of seventh grade students, examined the effectiveness of the multi-dimensional approach in teaching density, investigated the relations between prior student characteristics and their post-instructional understanding, and investigated if the concept of density develops as part of a conceptual system. Program description: In the first part of the study, student understanding of density was assessed in regular classrooms. In the second part, the investigator and a science teacher co-taught the density unit over a two-week period emphasizing relations between density, mass, volume, part-whole relations, and a scientific particulate conception of matter. A conceptual change approach was used which emphasized multiple representations of knowledge and the use of analogies. Sample: The sample in regular classes consisted of 1,645 seventh graders in 51 schools in the West Bank, Palestine. The intervention group consisted of 29 students in one school. Design and methods: The post-instructional understanding of density in 51 regularly taught classrooms was assessed in the first part of the study using a pencil-and paper test. In the second part, a pre-test was used with the intervention group. Students in both parts of the study took the same post-test. Descriptive statistics were calculated to describe student performance. Comparison between pre-test and post-test performance of students in the intervention group was conducted using "t"-test and ANOVA. Correlations between pre-test sub-scores and post-test scores for students in the intervention class also were calculated. ?[superscript 2] was used to test for co-development of the density concept and other concepts using the different items of the post-test for all groups. Results: Student understanding of density was found poor after instruction, while the intervention had a moderate effect on understanding. Students who started with a basic understanding of some aspects of density gained more from the intervention. The density concept co-developed with the concept of volume and a particulate conception of matter. Conclusions: Teaching density as part of a conceptual system helps promote understanding of the concept. This requires the continuous development and refinement of a learning progression of density, volume, and the particulate nature of matter on the one hand, and an in-depth treatment while teaching the concept on the other hand.