A common element of all views of relationship marketing is the “co-operate-to-compete” thesis. That is, to be an effective competitor often requires one to be an effective co-operator. One implication of this thesis is that not all instances of firms co-operating with each other constitute anti-competitive collusion. This article argues that, although neoclassical, perfect competition theory cannot provide a theoretical foundation for relationship marketing's “co-operate-to-compete” thesis, the recently developed “resource-advantage” theory of competition can do so. Furthermore, this article uses resource-advantage theory to address the relationship portfolio conundrum. Specifically, the paper argues that firms should develop a relationship portfolio that is comprised of relationships that constitute relational resources.