This paper discusses in what sense engineering knowledge is peculiar, in comparison with science. First, I trace the development of “epistemology of engineering knowledge” and introduce Walter Vincenti's analysis of normal design knowledge as one of the representative works in the literature. I propose four peculiarity theses of engineering knowledge as possible interpretations of his position. However, there are problems with these theses. First, scientific knowledge seems to have similar features as the ones Vincenti regards as peculiar to engineering knowledge. Second, in the context of the debate on the peculiarity of the engineering knowledge, Vincenti's approach seems to miss the point. We need a broader notion of peculiarity of engineering knowledge.