Research on conceptual change is still a powerful framework for empirical investigations on students' learning of science. During recent years, an increasing body of research has explored students' concepts prior and subsequent to instruction, leading to an extensive documentation of students' concepts including the difficulties and opportunities of teaching for conceptual change. Though the research on conceptual change can inform practitioners about students' ideas of science and students' learning difficulties, little is known about the processes of conceptual development and students' use of developed concepts. Exploring the nature of the processes may help to understand what kind of concepts appear plausible and fruitful to students and how students apply their knowledge to scientific tasks. In order to investigate processes of the development of concepts, videoing a total of 45 students from grades 8 and 11 was carried out while they were working on physics tasks in small groups. The dimensions of content, level of abstraction, and time were used to describe the quality of students' situated knowledge as well as usage and changes of the knowledge when students were confronted with similar tasks. Assuming the notion "concept" refers to rule-based knowledge, the dimension of abstraction was used to distinguish between concrete (non-conceptual) and rule-based (conceptual) knowledge. Results on the processes indicate that most of the high school students' activities did not refer to conceptual knowledge. Moreover, explanations based on physics concepts which were offered to the students were rarely used. Furthermore, if students came up with conceptual descriptions of occurring phenomena, these descriptions showed a high variability in their content.