Background: Uncertainty is a crucial element of scientific knowledge growth. Students should have some understanding of how science knowledge is developed and why scientific conclusions are considered more or less certain than others. A component of the nature of science, it is considered an important aspect of science education and allows students to recognize the limitations of scientific research. Purpose: This study examined Grades 5 and 9 students' views of uncertainty in their personal scientific research and the formal scientific research of professionals. Sample: This study included 33 students in Grade 5 (n = 17) and Grade 9 (n = 16). The students were recruited from a charter school that emphasised inquiry instruction. Design and methods: Data were collected through interviews. Students were asked their views of their inquiry-based projects and their views of professional science. Results: Interview data and statistical analyses indicated that students recognized uncertainty in personal science, which varied across elements of the scientific process. Additionally, their views of uncertainty in formal science tended to change across grades and knowledge of uncertainty in personal and formal science were positively correlated. Conclusion: These findings offer insights into the processes by which students come to understand uncertainty in science and point to ways of fostering such knowledge through teaching practices.