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Several states and transit agencies have recently introduced zero-emission buses (ZEBs) to their fleets, including battery electric buses, fuel cell battery electric buses, and fuel cell plug-in hybrid electric buses in an effort to mitigate transportation-related impacts on air quality. The objective of this paper was to summarise information and insights from ZEB implementations across the United States (U.S.) to advance the state of practice and facilitate transit agencies' transition to ZEBs. Information was obtained through a critical review of relevant literature from peer-reviewed journals and agency reports released by transit agencies and other relevant stakeholders, an online survey of several transit agencies that implemented or are planning to implement ZEBs, and interviews with transit agency representatives and other relevant stakeholders. This review focuses on ZEB in-service performance, cost, fuelling, and implementation strategies. In addition, challenges and lessons learned as reported by U.S. transit agencies are described. A comparison among the three technologies and conventional fuel buses (i.e. diesel and compressed natural gas) suggests that ZEBs outperform conventional buses in terms of fuel efficiency, but their procurement cost is higher. Battery electric buses present the highest fuel efficiency and lowest procurement, operation, and maintenance costs, and have been chosen by most transit agencies followed by fuel cell battery electric buses. While fuel cell hybrid plug-in buses have been implemented by several agencies over the past ten years, they have not gained popularity due to increased purchase and maintenance costs. Important factors for agencies to consider when implementing any of these ZEB technologies include: (1) fleet size: starting with a small fleet and gradually expanding; (2) technology type: understanding the technology and properly choosing the one that matches the needs and limitations of a service area (e.g. available space, existing schedule) as well as available resources and agency goals; (3) staff training: proper training for a suitable amount of time of drivers and maintenance personnel; and (4) stakeholder collaboration: having an effective level of collaboration, cooperation, and support between stakeholders. Overall, ZEBs are a viable and promising approach for reducing emissions from transit fleets. This review can assist in transit agencies' transition to ZEB fleets by providing useful information and insights to ensure optimal technology choice and efficient implementations. It can also provide insights on ZEB implementation issues that warrant further research. [ABSTRACT FROM AUTHOR]