This paper presents a series of structural models representing the acquisition of writing language and the performance and mastery of reading and writing tasks. The first aim of our paper is to illustrate - by using examples from actual situations - the usefulness that structural models have for researchers in the area of written language. The application of these models concerns both the design of the data collection strategy and the data analysis procedure. A second purpose is to show the importance of the analysis of functional relations between variables influencing the acquisition and mastery of writing and reading tasks. This analysis suggests going beyond the assumption of causal relations between two or more variables in studying written language, when formal education starts.