"A Framework for K-12 Science Education," the foundation for the Next Generation Science Standards (NGSS), identifies scientific explanation as one of the eight practices "essential for learning science" (National Research Council, 2012, p. 41). In order to design professional development so teachers can implement these new standards, we need to assess students' current skill levels in explanation construction, characterize current teacher practice surrounding it, and identify best practices for supporting students in explanation construction. This case study investigated teacher practice in two high school science inquiry units in the Portland metro area and the scientific explanations the students produced in their work samples. Teacher Instructional Portfolios (TIPs) were analyzed qualitatively based on best practices in teaching science inquiry and a qualitative coding scheme. Written scientific explanations were analyzed with an explanation rubric and qualitative codes. Relationships between instructional practices and explanation quality were examined, and five factors that support students in producing scientific explanations that align with the NGSS were identified: (1) strong content knowledge regarding the theory underlying the science inquiry investigation, (2) balanced pedagogical techniques, (3) previous experience conducting science inquiry, (4) an open-ended investigation topic, and (5) clear expectations for explanation construction aligned with relevant standards.