The study of adolescent perceptions of their ethnicity/race has been investigated from an ethnic-racial identity (measurement) perspective or through an ethnic-racial classification lens. This study examines both, as it explores change in adolescents' ethnic-racial self-categorization; change in strength of ethnic-racial identity (ERI); and the relationship between change in self-categorization and strength of ERI. In so doing, it contributes to theorizing about the content and process of ERI formation. The longitudinal and mixed-methods study followed 732 middle and high school students biannually in a semi-rural, ethnically diverse school district. All students completed the Multigroup Ethnic Identity Measure (MEIM) in fall and spring each year and interviewed annually for qualitative inquiry. The four waves of data demonstrated that: (1) 22% of students changed their ethnic-racial classification at least once; (2) Multiracial youth, boys, and 8th graders were significantly more likely to change self-classification compared to White youth, girls, and 10 th graders; (3) strength of ERI did not change over time, however, African American and Hispanic youth had the strongest ERI over time compared to the others; (4) change in self-classification was significantly related to change in strength of ERI, however, its direction varied by race of students. While adolescent ethnic-racial identity development (ERI) appears to be related to ethnic-racial categorization, the direction of the relationship varies depending on ethnic self-categorization. Results contribute to theory building of ERI in relation to: content and process, group consciousness, certainty and clarity, and public regard.