As managers of extensive vehicle fleets and transportation infrastructures, public transit agencies present unique opportunities for reducing greenhouse gas (GHG) emissions from the transportation sector. To achieve substantial and cost-effective GHG emissions reductions from their activities, public transit agencies need tools and resources that enable effective GHG emissions management. This research thesis presents the background, methodology, and results of the author's development of a public transit agency-level life cycle GHG emissions calculator. The development of the calculator involved a series of research efforts aimed at identifying and addressing the needs of transit agency GHG emissions management: a review of background information on climate change and public transit's role in mitigating climate change; a review of existing GHG emissions calculators for public transit agencies, a review of the methodologies for life cycle GHG emissions analysis; integration and adaption of existing calculation resources; development of calculator spreadsheets for estimating relevant lifecycle GHG emissions and quantifying GHG emission reduction cost-effectiveness; application of the developed calculator to a carbon footprint analysis for a typical mid-size to large-size transit agency; and application of the developed calculator to the evaluation of the cost-effectiveness of various potential strategies for reducing transit agency GHG emissions. The developed calculator provides an integrative resource for quantifying GHG emissions and costs of public transit agency activities, including GHG emission reduction strategies. Further research is needed to calibrate the estimation of upstream life cycle GHG emissions, particularly for vehicle manufacture and maintenance.