Tobacco use continues to be the leading preventable cause of mortality and morbidity in North America and Quitlines are one of the primary cessation resources available to assist tobacco users with quitting. Implementation of best practices is important to the success of quitlines, but unfortunately, it is a complex and elusive process often difficult to achieve. This study aims to better understand the implementation process by using qualitative methods to examine an evaluation practice in-depth and to elucidate the complex factors influencing its implementation and institutionalization in the North American Quitline network. Nineteen semi-structured interviews were conducted with decision-makers in the Quitline network. The interview data were analyzed using a thematic analysis approach, guided by a systems change framework. The findings suggest that a broad range of factors influenced implementation of the evaluation practice at different levels of the system. These factors included system norms, system resources and operations (i.e., policies), five key relationships (e.g., between the funder and service provider), and power placement in the system. Characteristics of the evaluation practice itself also influenced implementation and interacted with other factors in the system. This study demonstrates the complexity of implementing and institutionalizing evaluation in an inter-organizational network. It also demonstrates the value of using qualitative data to study implementation phenomena. The findings can be used to improve efforts to institutionalize evaluation in the Quitline network and inform future implementation research studies. (Contains 3 tables and 1 figure.)