This article examines prospective elementary teachers' conceptions of unitizing with whole numbers and fraction concepts and operations throughout a semester-long mathematics content course. Student work samples and classroom conversations are used to illustrate the types of unitizing understandings that prospective teachers bring to teacher education programs. The results indicate that the prospective teachers were successful with iterating units and developing composite units within both whole numbers and fractions. Although this did not indicate they had a complete understanding of unitizing, the results imply that tools, such as number lines, may be useful in determining what understandings they do have.