Knowledge of students’ unscientific understanding before learning a new topic known as students’ preconception or prior knowledge is vital for helping the teacher design a proper teaching strategy. Meanwhile, knowledge of students’ understanding after teaching will provide a way for a teacher to evaluate the effectiveness of his/her teaching. For these reasons, science educators should investigate students’ understanding over time. Studying students’ understanding requires a proper and powerful tool/instrument such as a multi-tier instrument. This paper describes the history of multi-tier instruments initiated by the two-tier and recently became a five-tier instrument, the procedure to develop the instrument, and how to utilize the instrument to identify students’ unscientific understanding. Our recent study describing the development of a four-tier instrument of electrolyte and non-electrolyte solution (FTI-ENES) is presented.