Theoretical models hold that written products (e.g., quality of written composition) are the outcome of the writing process (e.g., translation, transcription, revision) and skills and knowledge on which the writing process draws (e.g., language, transcription, cognitive skills). In the present study, we examined the relations among writing quality; the writing production process measured by writing fluency; and language, cognitive, and transcription skills, using longitudinal data from English-speaking beginning writers who were followed from kindergarten to Grade 2 (N = 261). Children's working memory, attentional control, spelling, handwriting fluency, vocabulary, grammatical knowledge, and writing quality were measured in kindergarten, Grade 1, and Grade 2. Writing fluency was measured in Grade 2 in two ways, using a process-based measure, writing burst length (chunks of text produced between pauses during writing), and a product-based measure, the number of words produced per total writing time. Results from structural equation modeling showed that Grade 2 writing fluency was moderately related to Grade 2 writing quality (.40) and completely mediated the relations of Grade 1 spelling and handwriting fluency skills to Grade 2 writing quality. In contrast, Grade 1 vocabulary was directly related to Grade 2 writing quality over and above Grade 2 writing fluency. Kindergarten working memory was indirectly related to Grade 2 writing fluency via Grade 1 spelling skill. These findings indicate that the writing production process measured by writing fluency mediates the relations of transcription skills to writing quality, and executive function is indirectly related to writing fluency via transcription skills for beginning writers. Educational Impact and Implications Statement: This study advances our understanding of how a written product, writing quality, is predicted by the writing production process measured by writing fluency as well as language, cognitive, and transcription skills. We measured young beginning writers' writing fluency by a process-based measure, writing bursts (chunks of text produced between pauses while writing) using a digital pen with an infrared camera at its nib, as well as a product-based measure, the number of words produced per total writing time. Results showed that children who had higher writing fluency—those who produced longer writing bursts (who can write more words and letters per burst) and wrote a greater number of words per total writing time—had higher quality writing, and this was explained directly by transcription skills and indirectly by executive functions such as working memory. [ABSTRACT FROM AUTHOR]