In this study, we investigated students' mental models of wave-particle duality. We interviewed six engineering and premedical students enrolled in a calculus-based physics course. We conducted a qualitative examination of student interviews and found that students displayed four different mental models: quantum model (QM), semi-quantum model (SQM), wave model (WM), and in-between model (IM). We also found that 2/3 of the students had difficulty discriminating between classical and quantum ideas and held mental models that were inconsistent and context-specific. This work provides a much-needed detailed exploration of students' conceptions of quantum theory, which will help instructors understand conceptual barriers frequently encountered by introductory students.