Falsehood can preclude knowledge in many ways. A false proposition cannot be known. A false ground can prevent knowledge of a truth, or so we argue, but not every false ground deprives its subject of knowledge. A falsehood that is not a ground for belief can also prevent knowledge of a truth. This paper provides a systematic account of just when falsehood precludes knowledge, and hence when it does not. We present the paper as an approach to the Gettier problem and arrive at a relatively simple theory with virtues linked to several issues at the heart of contemporary epistemology.