In addition to motor learning to form letters, linguistic and cognitive demands quickly increase for Western Australian students learning to handwrite in Year 1 and Year 2. However, current handwriting assessment instruments focus mainly on the motor learning demand and rarely incorporate the linguistic and cognitive demands of handwriting tasks. As a consequence, when children are diagnosed as having handwriting problems, the focus is invariably on the motor learning demand, even when their difficulties arise from the linguistic and cognitive demands. The lack of an assessment that takes into account these demands undervalues the contribution of handwriting legiblity to early writing development. Three studies were undertaken. The purpose of Study One was to construct an assessment instrument to measure handwriting legibility that incorporated not only the motor demand, but also the linguistic and cognitive demands. Three hierarchically ordered writing tasks of increasing cognitive load, defined by task complexity, were devised. It was predicted that increasing cognitive load would result in apparent deterioration of handwriting performance. To validate the instrument, students in Year 1, Year 2 and Year 3 (n=417) from private and government schools were recruited using a cross-sectional design. Classroom teachers administered far-point copying, dictation, and composition tasks on consecutive days at the end of first term. Handwriting legibility for each task was scored by the researcher on six aspects of legibility using a marking rubric: letter formation, size, space in word, space between words, line placement and slant. The rubric demonstrated acceptable inter- and intra-rater reliability. The hypothesis of apparent deteriorating performance as linguistic and cognitive demands were increased was confirmed from a qualitative analysis, and a quantitative analysis using the Rasch measurement model to validate the Handwriting Legibility Scale (HLS). It was shown that the Rasch model accounts for the relative difficulty of copying, dictation and composition tasks, and thereby permits a more accurate assessment of handwriting legibility. In Study Two, a longitudinal study over two years, the Year 1 cohort provided four data sets that were anchored on the item locations of the HLS, confirming the validity of the scale and reliability of the marking key. A third study investigated the relationship between legibility on the HLS and automaticity on the Alphabet Task for Year 1 students. The findings suggest a trend from no relationship to a growing relationship. The evidence that learning to write has significant linguistic and cognitive load demands, not known before, has important implications for teaching writing, especially for those students who show difficulty in learning to write.