With this graduate thesis, I wanted to explore how familiar the elementary and high school students are with the concepts of the control of variables and models and how they apply these concepts in different contexts. In the test construction, I used test items from Project 2061 from the American Association for the Advancement of Science (AAAS) project website. The test consisted of 15 multiple choice questions where students had to apply different ideas related to control of variables and models. I conducted the research in Zagreb on 171 seventh and eighth grade elementary school students and 196 first and second grade high school students. By analyzing the data, I gained an insight into the test results for both elementary and high school, for each particular grade, the percentage of correct answers for each given task, and also the percentage of the most commonly chosen wrong answers which give an insight into the student misconceptions. Furthermore, I compared the results of Croatian and American students whose data I took from the AAAS Project 2061 website. The results showed that the students from elementary school scored lower than the student from high school, and that the second-grade students had the best test results. Additionally, the results showed that the students had greater progress in understanding control of variables than understanding of models, and that there are difficulties in understanding of some basic physics concepts that occur among students irrespective of whether they are in elementary or high school. On average, the Croatian students had somewhat better test results than the American students, however, this difference was apparent only for some test items. While teachers today are struggling to achieve active participation of students in classes, it is not easy or simple to change the misconceptions. The results of this graduate thesis suggest that more attention in teaching should be devoted to the meaning and use of models, and better understanding control of variables.