Latin-American governments are increasingly promoting public-private partnerships to provide much-needed external support to schools, but little is known about the mechanisms through which such partnerships operate. This study aims to contribute to the literature on external support for school improvement by analyzing how a non-government organization in Argentina provides support to schools using two different implementation mechanisms, one where instructional support is provided directly by the non-government organization to schools, and another where school support strategies are mediated by government district offices. The study is a qualitative exploration of the dynamics of the external support relationship. Using interviews, observations, and document review as sources of data, I explore how the institutional arrangements involved in different implementation mechanisms affect how the external support program is experienced at the district and school level. The analysis shows that the original design of the instructional support strategy developed by the program underwent significant adaptations as it was being implemented at the different levels of program implementation. These differences seem to have been related to the institutional choices that were made throughout program implementation, rather than to the interaction between the program and schools with different levels of internal capacity. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]