The Conceptual Survey of Electricity and Magnetism (CSEM) was developed to assess students' knowledge about topics in electricity and magnetism. The survey is a 32-question, multiple-choice test that can be used as both a pretest and posttest. During four years of testing and refinement, the survey has been given in one form or another to more than 5000 introductory physics students at 30 different institutions. Typical pretest results are that students in calculus-based courses get 31% of the questions correct and student's in algebra/trigonometry-based courses average 25% correct. Posttest correct results only rise to 47% and 44%, respectively. From analysis of student responses, a number of student difficulties in electricity and magnetism are indicated.