Scholarly work seeking to understand academics’ commercial activities often draws on abstract notions of the academic reward system and the representative scientist. Few scholars have examined whether and how scientists’ motives to engage in commercial activities differ across fields. Similarly, efforts to understand academics’ choices have focused on three self-interested motives—recognition, challenge, and money—ignoring the potential role of the desire to have an impact on others. Using panel data for a national sample of over 2,000 academics employed at U.S. institutions, we examine how the four motives are related to commercial activity measured by patenting. We find that all four motives are correlated with patenting, but these relationships differ systematically between the life sciences, physical sciences, and engineering. These field differences are consistent with differences across fields in the rewards from commercial activities as well as in the degree of overlap between traditional and commercializable research, which affects the opportunity costs of time spent away from “traditional” academic work. We discuss potential implications for policy makers, administrators, and managers as well as for future research on the scientific enterprise. This paper was accepted by Toby Stuart, entrepreneurship and innovation.