Many educational questions of research interest focus on individual differences in attitudes and behaviors related to academic achievement, changes in such attitudes and behaviors over time, and the types of academic environments that facilitate or prevent development of achievement attitudes and behaviors at school. This paper, the first in a 3-part series on modeling academic development, emphasizes the importance of studying intraindividual development and interindividual differences in intraindividual development. The paper presents a multilevel ecological model as a heuristic for exploring these types of educational questions and describes the use of various data analysis models for examining intraindividual academic growth patterns and interindividual differences in those patterns. It differentiates an incremental view and a process view of quantitative change and examines methods for analyzing longitudinal data related to student achievement. The paper concludes by noting that problems with measuring and analyzing change in academic achievement are derived from an inaccurate conceptualization of change. It is argued that research on academic development is challenging because an examination of student growth involves nested structures of repeated observations within individuals who in turn are nested within school settings. Hierarchical linear models are presented as an approach that can consider each student as a group, allowing repeated measures within each subject. Further, the paper shows that a growth curve approach in hierarchical linear models makes it possible to study intraindividual change and to examine which characteristics may be associated with interindividual differences in intraindividual change. (Contains 41 references.) (KB)