What is the relationship between lying, belief, and knowledge? Most philosophical accounts of lying define it in terms of belief, namely telling someone something that one believes to be false, often with the intent to deceive. This chapter develops a novel account of lying by deriving evaluative dimensions of responsibility from the knowledge norm of assertion. On these dimensions, we can distinguish reasonable assertions, negligent assertions, and vicious assertions. On the definition of lying defended here, lies are best understood as special cases of vicious assertion; thus lying is the anti-paradigm of proper assertion. This enables an account of lying in terms of knowledge rather than belief: roughly, lying is telling someone something you know ain’t so.