The ACER Longitudinal Literacy and Numeracy Study (LLANS) was designed to follow the growth in literacy and numeracy of a single cohort of students across the years of primary school. A sample of 1000 students in 100 schools formed the cohort for the study, which commenced in 1999. A linked set of literacy assessment tasks focused on key aspects of literacy was developed for the LLANS. Assessment data collected across the years of primary school provided information needed to construct a long LLANS literacy scale covering the full range of proficiency. The scale was used to measure change and track students' achievement over time in the early years of school. This article begins by describing the conceptual framework for the study, and the methodology. The linked set of literacy assessment tasks developed for the first three years of the study, and the long literacy scale are discussed. Findings about the progress of the whole cohort in the first three years, and about individual growth trajectories are presented. More detail about this phase of the study can be found in an ACER Research Monograph. Findings across the seven years of study will be reported in further research monographs.