Learning conceptual knowledge in engineering science is a critical element in the development of competence and expertise in engineering. To date, however, research on conceptual learning in engineering science has been limited. Therefore, this article draws heavily on fundamental research by cognitive psychologists and applied research by science educators to provide a background on fundamental issues in the field and methods for assessing conceptual knowledge. Some of the most common conceptual difficulties from three domains: mechanics, thermal science and direct current electricity, are discussed to provide concrete examples of what students find difficult to learn. The article concludes with a discussion of possible sources of these difficulties, implications for instruction, and suggestions for future research.