Elementary engineering education (EEE) is an educational innovation. Using Rogers's innovation diffusion model, the Concerns-Based Adoption Model (CBAM), and Dreyfus's skill acquisition model as its theoretical frameworks, this study investigated elementary teachers' EEE adoption and EEE expertise development. Data of this study were collected through face-to-face interviews and open-ended online surveys conducted with 73 elementary teachers who received one-week EEE training from INSPIRE, the Institute for P-12 Engineering Research and Learning at Purdue University. An analytic induction approach was adopted in the analyses of the data. Based on the data analyses results, an evidence-based EEE adoption and expertise development framework was constructed to describe the process of EEE adoption and EEE expertise development and to capture individual elementary teachers' differences in this process. This framework includes the four-staged EEE adoption dimension and the five-staged EEE expertise development dimension. This framework is able to reveal the "synchronic differences" and the "diachronic progression" in EEE adoption and EEE expertise development. While the "synchronic differences" indicate elementary teachers' different standings in the EEE adoption and EEE expertise development stages at a given time, the "diachronic progression" indicates progress along the stages over time. This framework is proposed to be used by EEE professional development programs to conceptualize, assess, and track their teacher learners' standings and progress in EEE adoption and EEE expertise development for the purpose of program improvement and the purpose of providing teacher learners with effective and on-going support.