This study explores undergraduate students' understanding and reasoning models of electric and magnetic fields. The results indicate that the tested students had various alternative concepts in applying their reasoning to certain CSEM test questions. The total number of physics students tested at the beginning of the first course on electromagnetism was 144, the overall percentage of correct answers being 31%. Based on the CSEM test results, five students were interviewed by using certain CSEM test questions in order to reach a deeper understanding of their reasoning models underlying their answers. (Contains 3 tables and 5 figures.)