This paper presents a research-based framework for analyzing and monitoring students' understanding of functions in equation form. The framework consists of "growth points" which describe "big ideas" of students' understanding of the concept. The data were collected from Grades 8, 9, and 10 students using a set of tasks involving linear and quadratic relationships, and were used in order to identify and describe "big ideas" in students' understanding of function in equation form. The framework also shows a typical learning path leading towards generality and abstraction in students' thinking about function. (Contains 2 tables and 1 figure.)