The Next-Generation Science Standards (NGSS) challenge primary teachers and students to work and think like scientists and engineers as they strive to understand complex concepts. Teachers and teacher educators can leverage what is already known about inquiry teaching as they plan instruction to help students meet the new standards. This cross-case analysis of a multiple case study examined teacher practices in the context of a semester-long professional development course for elementary teachers. We reviewed lessons and teacher reflections, examining how kindergarten and first grade teachers incorporated NGSS scientific and engineering practices during inquiry-based instruction. We found that most of the teachers worked with their students on asking questions; planning and carrying out investigations; analyzing and interpreting data, using mathematics and computational thinking; and obtaining, evaluating and communicating information. Teachers faced challenges in supporting students in developing their own questions that could be investigated and using data collection strategies that aligned with students' development of number sense concepts. Also, some teachers overemphasized "the scientific method" and lacked clarity in how they elicited and responded to student predictions. Discussion focuses on teacher supports that will be needed as states transition to NGSS.