We investigated six preservice secondary science teachers' implementation of reform-based science, in particular, their teaching of the "Next Generation Science Standards' (NGSS)" science and engineering practice of "using mathematics and computational thinking." A modified version of the Task Analysis Guide in Science served as our conceptual framework: It assesses both the "integration" of practices and content (i.e., the kind of thinking required), and the "cognitive demand" of tasks (i.e., the level of thinking required) in teachers' lessons. We used this framework to qualitatively analyze our preservice teacher participants' edTPA (teacher performance assessment) lessons--including their written commentaries, video-recorded lesson excerpts, and student work samples--for their implementation of the "NGSS using mathematics and computational thinking" practice. We examined (1) the integration of the mathematical content and practices outlined in the "Common Core State Standards for Mathematics" within the target NGSS practice, and (2) the cognitive demand of the mathematics in relation to science and mathematical practices. We found that four of our six preservice teachers implemented lessons that were integrated and cognitively demanding: These participants used the mathematics to move students' understanding of the science phenomena forward. However, the other two participants implemented lessons that integrated mathematical content and practices but were low in cognitive demand. We conclude with implications for how teacher education programs can better support preservice teachers' implementation of lessons that are both integrated "and" cognitively demanding so as to promote students' mathematical reasoning and scientific sensemaking.