This study examined the relative contributions of measures of Cattell-Horn-Carroll (CHC) cognitive abilities in explaining writing achievement. Drawing from samples that covered the age range of 7 to 18 years, simultaneous multiple regression was used to regress scores from the Woodcock-Johnson III (WJ III; Woodcock, McGrew, & Mather, 2001) that represent CHC broad and narrow abilities onto the WJ III Basic Writing Skills and Written Expression cluster scores. At most age levels, Comprehension-Knowledge demonstrated moderate to strong effects on both writing clusters, Processing Speed demonstrated moderate effects on Basic Writing Skills and moderate to strong effects on Written Expression, and Short-Term Memory demonstrated moderate effects. At the youngest age levels, Long-Term Retrieval demonstrated moderate to strong effects on Basic Writing Skills and moderate effects on Written Expression. Auditory Processing, and Phonemic Awareness demonstrated moderate effects on only Written Expression at the youngest age levels and at some of the oldest age levels. Fluid Reasoning demonstrated moderate effects on both writing clusters only during some of the oldest age levels. Visual-Spatial Thinking primarily demonstrated negligible effects. The results provide insights into the cognitive abilities most important for understanding the writing skills of children during the school-age years. (Contains 4 figures and 5 footnotes.)