The onset of the Next-Generation Science Standards [NGSS] has created a need for elementary teachers to develop purposeful, multi-dimensional formative assessments (FAs). In order to promote this process, we utilized teacher learning communities (TLCs) and practice-based professional development focused on key aspects of the reform while documenting how four teachers in rural elementary schools cyclically developed and revised several FAs during a two-year project. Utilizing a multi-case embedded case study design, primary data collected included multiple teacher-developed, classroom-embedded FAs (including student work samples), recurrent audio of conversations wherein teachers discussed the results of their FAs with peers in their TLC, and five teacher interviews. The findings, though not intended to be generalizable, detail how developing FAs helped two of the teachers expose both instructional and assessment-related vulnerabilities as they learned more about the NGSS and the value of purposefully eliciting their student's thinking in a multi-dimensional manner. Conversely, the other two teachers maintained a prior preference for developing FAs that targeted the memorization of science content--though they did provide students with more "space" to reveal their thinking via more open-ended prompts. This adjustment unfortunately caused numerous challenges for students, as they struggled to provide the "correct" responses the pair sought. Implications discuss how the connections between instruction and assessment can become more tightly intertwined whilst cyclically designing FAs, with an emphasis on the science and engineering practices. Recommendations for future research stemming from outside and within the classroom, as well as practitioner-based supports are discussed.