Incoherentism about vagueness is the view that vague expressions/concepts are incoherent due to their vagueness. This chapter elaborates on what incoherentism is, and defends a particular incoherentist view. It presents an overview of important arguments for and against incoherentism. Among arguments for the view are claims that it provides an attractive account of the nature of vagueness, and of the way in which vagueness is associated with indeterminacy. Among arguments against the view are claims that it presupposes a mistaken view on semantic/conceptual competence, and that the view sits ill with how ubiquitous vagueness is. The specific view defended is compared to the views of Michael Dummett, Terence Horgan and Peter Unger.