Public-private partnerships (P3) are outperforming other delivery methods in their ability to meet sustainability objectives. The main driver for decisions in any project has been seen as linked to a cost analysis. This research aims to determine why and how P3 projects are more likely to implement more effective sustainable decisions throughout a project's lifecycle. In this context, the decision-making is directed explicitly at the project team's reasoning for implementing sustainable practices beyond cost-effectiveness. The researcher generated questions to ask potential project teams to understand why and how P3 projects were better at implementing sustainable decisions. Sustainability was operationalized using Envision's framework. Interviews with project teams that utilized the P3 project delivery method and received the highest sustainability rating provided a first-hand account of the decision-making process. The interviews are analyzed utilizing framework analysis. The results will identify the motivations for implementing sustainability. The results suggest that the P3 contract structure is the most compelling reason these projects can implement sustainable decisions better than other project delivery methods. The written requirements from the contract documents or legislative requirements and the Contractor's desire to do a good job are other drivers for this increased sustainability. / Master of Science / The infrastructure in the United States is reaching the end of its useful life. These assets need significant investments to continue serving their original function. Various project delivery methods exist that either deliver a product or a service. Delivery methods such as design-bid-build or design-build focus on delivering an asset such as a bridge or road at the end of the contract. The public-private partnership (P3) delivery method focuses on delivering an asset and operating that asset for an extended period until the end of the contract, typically no less than 25 years. Building these assets sustainably will help drive down costs and increase useful life. Sustainability goals cover the economic, environmental, and social aspects of the project. The economic goals include providing a responsible cost-benefit to the users or taxpayers and lasting for an extended period. The environmental goals include minimizing the project's impact on the environment. The social goals include building the right project so that it solves the correct community issue. The P3 delivery method of delivering an asset and service has outperformed other delivery methods in sustainability. This paper explored the reasons that project teams make decisions to include sustainable choices throughout their project contract.