In this paper, we describe our learning progressions approach to early algebra research that involves the coordination of a curricular framework, an instructional sequence, written assessments, and levels of sophistication describing the development of students' thinking. We focus in particular on what we have learning through this approach about the development of students' abilities to generalize and represent functional relationships in a grades 3-5 early algebra intervention by sharing the different levels of responses we observed in students' written work and the percent of students situated at each level across different tasks.