Collaborative co-creation of advertising campaigns by agencies and clients is widely perceived to result in greater creativity and higher-quality advertising outcomes. This research examines three levels of collaboration in client–agency relationships. Environmental-level effects were found in that clients often had difficulty ceding control and trusting an outside agency. Yet when agencies are viewed as experts, clients come to trust them and better creative work results. At the dyadic level, the depth of information sharing between the agency and client was helpful. In stable client–agency relationships, client involvement need not be as extensive because the agency already understands the strategy. Interorganizational effects were also explored, but a surprise finding was an unexpected inverted U-shaped relationship between agency competency and effectiveness. Only at a moderate level of agency competency is the most effective advertising observed, which is indicative of a “dark side” of client–agency relationships. This study was based on qualitative interviews with 20 marketing clients and agency professionals from Australia and New Zealand. Quantitative results were based on 162 campaigns from the same region reported on by 60 brand managers.