One ingredient that may serve as a catalyst for writing development is changes in writing knowledge. This study assessed the veracity of two tenets underlying this proposal: that skilled writers are more knowledgeable than less skilled writers, and that individual differences in knowledge are related to writing performance. Both of these assumptions were supported. First, fourth-grade students who were more skilled writers were more knowledgeable than their less skilled peers. They knew more about how writing promoted school and later occupational success. They also were more knowledgeable about the role of substantive processes in composing, the use of substantive procedures when writing for a younger child, and the value of seeking assistance when experiencing difficulties. Second, for more skilled writers, writing knowledge was significantly correlated with their story writing performance. (Contains 5 tables.)