This thesis is about writing proficiency among students of secondary education. Due to globalization, the ability to express oneself in a language other than the first language (L1) is increasingly becoming a condition for educational success. In The Netherlands, this ‘other’ or second language (L2) is usually English. Although secondary school students are already quite able to express themselves in English, their L2 essays are often of lower quality than their L1 essays, in terms of language use, but also in terms of organization. The research reported in this thesis is aimed at explaining this quality difference by comparing L1 and L2 relations between essay quality and writing processes. Analyses of writing processes involved cognitive activities such as reading the assignment, process planning, content planning, evaluating and revising. Results show that, in general, cognitive activities are relevant to essay quality at different stages of task execution during L1 and L2 writing. This means that writers need to distribute their attention differently across task execution during L1 and L2 writing. However, additional analyses show that if students’ general language proficiency levels are included in the analyses, this L1/L2 contrast disappears. For students with high general L2 proficiency, the demands of L2 writing in terms of how often cognitive activities are applied during certain stages of task execution are similar to the demands of L1 writing.