Using date from 120,612 students at 26 four-year institutions, this report provides a description of STEM majors entering four-year postsecondary institutions and those who persist through four years of study. The results indicate that persisting STEM majors enter college with very high levels of pre-college academic achievement as measured by the ACT™ test, and they have distinct interests as measured by the ACT Interest Inventory. Meta-Analysis of 26 institutional studies indicate that these patterns are rather consistent across colleges. Within institutions, STEM majors enter college with higher levels of pre-college academic achievement than non-STEM majors; the largest differences are seen on ACT mathematics and ACT science scores. Furthermore, the gaps between the STEM majors and non-STEM majors generally become larger over four years of study. STEM majors also differ from non-STEM majors in regard to their measured interests. The creation of two STEM categories, STEM-Biological and STEM-Quantitative, was supported by the differences seen in their ACT mathematics and science test scores and their measured interests. More young students are being encouraged to enter STEM fields. Although this trend is commendable, it is important for students to understand the high levels of academic achievement that persisting STEM majors achieve before entering college and the alignment of their interests with their choice of major. (Tables are appended.) [For the 2017 follow-up report, "Profiles of High-Performing STEM Majors. ACT Research Report Series 2017-2," see ED573720.]