This paper reports on the nature of the conceptual understandings developed by Year 12 Victorian Certificate of Education (VCE) physics students as they made the transition from the essentially deterministic notions of classical physics, to interpretations characteristic of quantum theory. The research findings revealed the fact that the understandings achieved by students were not always scientifically valid, or necessarily consistent with what had been taught in class. Analysis of the research data also revealed the fact that some of the students' mental constructions were in fact quite complex--often making use of a range of visual models and mathematical theories to make sense of quantum reality. This area of research is of significant importance given the fact that quantum theory plays such a central role in our understanding of science and technology.