Conceptual engineers aim to revise rather than describe our concepts. But what are concepts? And how does one engineer them? Answering these questions is of central importance for implementing and theorizing about conceptual engineering. This paper discusses and criticizes two influential views of this issue: semanticism, according to which conceptual engineers aim to change linguistic meanings, and psychologism, according to which conceptual engineers aim to change psychological structures. I argue that neither of these accounts can give us the full story. Instead, I propose and defend the Dual Content View of Conceptual Engineering. On this view, conceptual engineering targets concepts, where concepts are understood as having two kinds of contents: referential content and cognitive content. I show that this view is independently plausible and that it gives us a comprehensive account of conceptual engineering that helps to make progress on some of the most difficult problems surrounding conceptual engineering.