In defending integrative promise, I regularly appeal to two senses of integration: a horizontal sense, describing integration across the tree of life, and a vertical sense, describing integration across something like levels of organization. Despite the intuitive appeal of these notions, the idea of vertical integration in particular has been subject to much philosophical criticism. In this chapter, I detail that criticism, in the process building a modified definition of what I will call “inter-domain” integration that, I claim, can capture much of the intuition behind vertical integration without falling prey to the kinds of well-known metaphysical worries to which other forms of vertical integration are subject. Inter-domain integration, by focusing on the ways in which scientists foreground and background different kinds of explanatory resources, more naturally functions as an element of the pattern of explanatory virtues that is integrative promise.