Objective: The present study examines the structure and operation of social networks of information and advice and their role in making decisions whether to adopt new evidence-based practices among agency directors and other program professionals in 12 California counties participating in a large RCT. Methods: Interviews were conducted with 38 directors, assistant directors and program managers of county probation, mental health, and child welfare departments. Ground theory analytic methods were used to identify themes related to EBP adoption and network influences. A web-based survey collected additional quantitative information on members of information and advice networks of study participants. A mixed-methods approach to data analysis was used to create a sociometric dataset (n = 176) for examination of associations between advice seeking and network structure. Results: Systems leaders develop and maintain networks of information and advice based on roles, responsibility, geography, and friendship ties. Networks expose leaders to information about EBPs and opportunities to adopt EBPs; they also influence decisions to adopt EBPs. Individuals in counties at the same stage of implementation accounted for 83% of all network ties. Networks in counties that decided not to implement a specific EBP had no extra-county ties. Implementation of EBPs was associated with the size of county, urban vs rural counties, and in-degree centrality. Collaboration is viewed as critical to implementing EBPs, especially in small, rural counties where agencies have limited resources on their own. Conclusions: Successful implementation of evidence-based practices requires consideration and utilization of existing social networks of high status systems leaders.