This study compared and evaluated three alternative theoretical models of the readingwriting relationship. The reading dimensions of these models included word analysis, vocabulary, and sentence and passage comprehension components. The writing dimensions included spelling, vocabulary, sentence structure, and story organization components. The models differed with respect to the sequential orderings of relationships of the reading and writing dimensions. The interactive model permits the use of reading knowledge in writing, as well as the reverse. The other models (reading-to-writing; writing-to-reading) only allow knowledge to move in a single direction. These models were evaluated on their ability to account for the relationships found in an extensive corpus of reading and writing data collected from 256 second graders and 251 fifth graders (Shanahan, 1984). The interactive model fit the data better than did the reading-to-writing model at the second-grade level and the writing-to-reading model at both grade levels, and the reading-to-writing model was superior to the writing-to-reading model. Specific relationships are examined, and pedagogical and research implications are drawn.