This paper discusses the distinction between contributory expertise and interactional expertise developed by Harry Collins and Robert Evans over a series of publications. The distinction has been widely adopted and used to analyze the expertise of different groups of people, such as scientists, journalists and color blind. While the distinction is a fruitful tool to begin thinking about expertise in a more structured way, one also finds substantial inconsistencies and unnecessary vagueness in Collins and Evans' writings. Clarifying these issues will make the distinction an even more useful tool for analyzing expertise in science.