This study explored how four elementary teachers assessed the developmental appropriateness and importance of nine nature of science (NOS) aspects after participating in a yearlong professional development program. A multiple-embedded case study design was employed. The primary data sources included (a) Views of Nature of Science Elementary School Version 2 (VNOS-D2) questionnaire (Lederman & Khishfe, 2002), (b) Ideas about Science for Early Elementary (K-4) Students questionnaire (Sweeney, 2010), and (c) follow-up semi-structured interviews. Data were analyzed using Yin's (1994, 2003) analytic tactics of pattern matching, explanation building, and cross-case synthesis. The cross-case analysis revealed that our participants used the following criteria separately or in some combination when they were asked to rate NOS aspects in terms of developmental appropriateness and importance: (a) teachers' NOS learning experience, (b) NOS teaching experience, (c) knowledge of their students, (d) knowledge of curriculum, (e) knowledge of school context, and (f) perceptions about the utility value of a NOS aspect or a myth about a NOS aspect. We found that even though our participants did not rank all NOS aspects equally, they considered all nine NOS aspects developmentally appropriate and important enough to be introduced at the elementary level.