Cataloged from PDF version of article. Thesis (M.S.): Bilkent University, The Program Of Curriculum And Instruction, İhsan Doğramacı Bilkent University, 2021. Includes bibliographical references (leaves 111-117). The purpose of this study was to investigate high school students understanding of inertial and non-inertial reference frames. To this end the study used a test that was composed of two parts A and B. Part A consisted of 7 open-ended and part B 12 force multiple choice test questions. After obtaining the necessary permissions from the Ministry of Education, Turkey, the test was applied to a total of 301 9th, 10th and 12th grade high-students in 2019. The female and male ratio were balanced and resembled the Turkish national distribution. After data collection, the first step was to determine the categories based on the student responses of the open-ended questions. As a result of these procedure 40 categories were determined. Then OLAP Cube procedures were used to obtain descriptive statistics on the 40 emerged categories. Classical item analysis was conducted on the force multiple choice part of the test. The Cronbach alpha value was found to be 0.53 and the test was classified as difficult. As a result of the analyses, we determined four major student understanding difficulties. These were labeled as In a lab/ground frame versus no frame/relativity, rotating versus steady objects, rotating frame and Newton’s law. by Ece Güneysu M.A.