The United Nations Educational, Scientific, and Cultural Organization promoted the creation of a model instrument for individual assessment of students’ foundational writing skills in the Spanish language that was based on a literature review and existing writing tools and assessments. The purpose of the Early Grade Writing Assessment (EGWA) is to document learners’ basic writing skills, mapped in composing units of increasing complexity to communicate meaning. Validation and standardization of EGWA was conducted in the Canary Islands (Spain) in 12 schools using a cross-sectional design with a sample of 1,653 Spanish-speaking students in Grades 1 through 3. The author describes EGWA’s internal structure, along with the prevalence of learning disabilities (LD) in transcription and developmental differences in writing between Spanish-speaking children with LD and typical peers. Findings suggest that EGWA’s psychometric characteristics are satisfactory, and its internal structure can be attributed to four factors responsible for a high percentage of the variance. The odds ratio indicated that 2 Spanish-speaking children with LD in transcription are identified out of every 100. A comparison between students with and without LD in transcription revealed statistically significant differences concerning sentence and text production across grades. Results are interpreted within current theoretical accounts of writing models.