This article examines the ability of frontline human service agencies to collaborate across organizational boundaries. Data come from an in-depth study of the public welfare and private welfare-to-work contractors in two Michigan counties and document significant problems that arise from the inability of these two sectors to collaborate in the provision of welfare programs. I use ethnographic methods that capture the perspectives of frontline workers themselves in order to understand how collaboration is actually thwarted. In spite of dramatically different organizational settings, frontline staff in both sectors draw on the same sources of evidence, that is, past relations, daily experiences, and client stories, when assessing the organizations with which they are mandated to collaborate. These collective beliefs create parameters within which staff interpret events and react to them. Their interpretations and reactions further reinforce the beliefs shared throughout the organization about the legitimacy, efficiency, and effectiveness of the partner organization. In this way, the social process at the front lines of the welfare system creates systemic barriers to collaboration. I conclude by considering how this analysis can help the local manager improve policy implementation and human service collaboration.