We investigate the difficulties that students in calculus-based introductory physics courses have with the concepts of symmetry, electric field, and electric flux which are important for applying Gauss's law. The determination of the electric field using Gauss's law requires determining the symmetry of a particular charge distribution and predicting the direction of the electric field everywhere if a high symmetry exists. Effective application of Gauss's law implicitly requires understanding the principle of superposition for electric fields. Helping students learn when Gauss's law can be readily applied to determine the strength of the electric field, and then helping them learn to determine the appropriate shape of Gaussian surfaces if sufficient symmetry exists, can help develop their reasoning and problem-solving skills. We administered free-response and multiple-choice questions and conducted interviews with individual students using a think-aloud protocol to elucidate the difficulties that students have with the concepts of symmetry, electric field, and electric flux. We also developed a multiple-choice test that targets these conceptual issues to obtain quantitative information about their difficulties and administered it to 541 students in the introductory calculus-based physics courses and to upper-level undergraduates in an electricity and magnetism course and to graduate students enrolled in a teaching assistant seminar course. We find that undergraduate students have many common difficulties with these concepts.