In business-to-business (B2B) selling, salespeople are entrusted with acquiring and managing one of the organization’s most important assets, customers, with the purpose of building and maintaining longterm, profitable relationships (Palmatier, Scheer, & Steenkamp, 2007). To accomplish this, organizations have invested significant amount of time and resources as > 3.6 million salespeople in the U.S. are involved in B2B selling (Zoltners, Sinha, & Lorimer, 2008) which accounts for approximately $22.7 trillion in 2016 (Skousen, 2016), more than the U.S. gross domestic product (~$18.2 trillion). Consequently, researchers have also been increasingly attentive to the B2B selling context, mainly to understand the underlying mechanisms and processes to build and maintain business relationships (Palmatier, Dant, & Grewal, 2007). An important factor in the B2B selling research stream is cooperation—[defined as the coordinated and complementary actions between the partners in an exchange to achieve mutual goals (Palmatier, Dant, Grewal, & Evans, 2006, p. 139]—given its central role in building strong business relationships (Anderson & Narus, 1990; Morgan & Hunt, 1994; Yen & Barnes, 2011). Considering its importance to the practice and the literature, our research investigates B2B relationships from the perspective of cooperation between salespeople and their customers in order to better understanding the process through which salesperson cooperation in- fluences business relationships. Research on salesperson-customer cooperation indicates that cooperation is a critical, if not the most important, factor for success in relationships (Palmatier et al., 2006). This is significant because cooperation can complement each firm’s weaknesses and enhance its strengths (Jap, 1999), thereby providing both firms with increased ef- ficiency and performance (Kim, Kim, Pae, & Yip, 2013). In fact, cooperation leads to mutual outcomes (with expected reciprocation over time) that exceed what one would achieve if each firm acted solely on its own (Anderson & Narus, 1990). This is at least partially because a cooperative salesperson—with his or her collaborative-focused approach—can use complementary actions with his or her customers to achieve mutual outcomes, with expected reciprocation over time. In spite of the importance and promise of cooperation in B2B relationships, empirical research in this area has lacked consistency. Specifically, findings of results from salesperson-customer cooperation on positive relationship outcomes have varied greatly, with effect sizes ranging from 0.24 to 0.73. The lack of consistency in findings creates a problem for managers to assess the true importance of how cooperation impacts relationship outcomes. A possible cause for this lack of consistency is that cooperation has been investigated from one side of the relationship or the other, from customers or salespeople, but not from both (with one exception, Anderson & Narus, 1990). Even though cooperation involves both parties, a dyadic perspective has also been mostly overlooked in the literature. A dyadic approach will provide insight on why and when cooperation reported by a salesperson or perceived by the customer translates to positive outcomes. In our research, we aim to address the inconsistent findings in prior literature by disentangling views of cooperation from both the salesperson and customer. In doing so, we explore why and when cooperation in B2B selling impacts relationship outcomes.