One dimension of cognitive success concerns getting it right: having many true beliefs and no false ones. Another dimension of cognitive success concerns using the right concepts. For example, using a concept of a person that systematically excludes people of certain demographics from its extension is a sort of cognitive deficiency. This view, if correct, tasks inquirers with critically examining the concepts they are using and perhaps replacing those concepts with new and better ones. This task is often referred to (...) as “conceptual engineering”. However, so far it is unclear what exactly happens in cases of conceptual engineering. How does language change when we engineer a concept? This article offers an answer. I propose a view on which, when speakers assess the truth of propositions, they often rely on assumptions with regard to what is required for their truth. For example, when speakers assess whether unborn fetuses are people, they rely on assumptions with regard to what is required to be a person. Based on this idea, I develop a model of conceptual engineering according to which speakers “engineer concepts” when they change how they assess the truth of propositions. For example, speakers engineer the concept of a person when they change how they assess the truth of the proposition that unborn fetuses are people. (shrink)