Fluency is an essential component in writing ability and development. Writing fluency research is important to researchers and teachers interested in facilitating students' written text production and in assessing writing. This calls for reaching a better understanding of writing fluency and how it should be measured. Although fluency is the construct with the most varied definitions and measures in writing research, such large variance in conceptualizing the construct is rarely discussed. In an attempt to demystify the what of this construct, the present article reviews its definitions, shows how its measurement has been influenced by oral production research, and discusses some issues related to the validity of the varied measures used for assessing it.