K-12 science teachers in the United States are encouraged to teach their students engineering. When incorporating engineering into their science curricula, teachers commonly either focus on: (1) engineering and lace science throughout; or (2) science and lace engineering throughout. This study explores middle school students' nature of engineering understanding before and after instruction with different engineering foci through the collection and analysis of student surveys (n~200) and interviews (n = 16). Findings demonstrated centering science while lacing engineering throughout allowed for a slight increase in the understanding that engineers use science and math, however, centering engineering allowed students to understand many more attributes of engineers (i.e., engineers invent/design/create, improve things, fix, and solve problems) after instruction than they did prior to instruction. These findings and their limitations are discussed in relation to the developing Nature of Engineering framework and the new content demands of the U.S. Next Generation Science Standards.