This report presents a comprehensive business case analysis of the Central Artery Tunnel (Big Dig) project. It begins with an executive summary and then delves into the project's objectives, critical success factors, and options analysis, evaluating various alternatives such as doing nothing, building a rail connection, and constructing an underground tunnel. The report recommends the tunnel construction option, highlighting its benefits in terms of congestion relief, accident reduction, and economic improvement. It then defines the project's scope, constraints, deliverables, and key performance indicators (KPIs). The project approach section outlines the project management strategy, including project structure, governance framework, reporting structure, procurement strategy, and communication strategies. Budget, program, and risk analyses are also included, along with a benefits realization plan. The report concludes with references and appendices, providing a detailed overview of the project's planning, execution, and management.