The aim of this project is to develop a tool for assessing the biological conceptual understanding of secondary grade-level students (grades 7 to 12) that is reliable and valid. Currently, there is no such assessment that has been developed for secondary students. In the development of the Secondary Biology Concept Inventory (S–BCI), tasks were designed to target major biology constructs taught during these grade levels. The assessment items answer stems were developed to include distractors representing students’ alternative conceptions obtained from literature and student interviews. The validation stage of the S–BCI development included (i) student interviews and (ii) multi-expert panel critique. The multi-expert panels included a panel of expert biologists and a separate panel of expert teachers. The reliability stage included S–BCI pilot-testing with a large sample of students in grades 7 to 12.