Condition assessment of subway stations is a major issue facing public transit authorities worldwide. The principal obstacle to the development of effective planning strategies is the lack of condition assessment models of subway stations. This books introduces a new methodology for the condition assessment of subway stations, entitled, the 'Subway Station Diagnosis Index (SSDI)'. The SSDI model is used to diagnose a specific subway station and assess its condition using an index (0 to 10). Based on the SSDI, the condition scale describes the station's condition state, its deterioration level (%), and proposed consequent actions. The new methodology identifies and evaluates the different functional criteria for subway stations; mainly structural, architectural, mechanical, electrical, security and communications. It uses specific multi-criteria decision analysis tools in order to evaluate a 'Functional Diagnosis Index' (FDI), and a global 'Station Diagnosis Index' (SDI). This book is relevant to industry practitioners (management, engineers, and field inspectors) and researchers, since it presents a multi-criteria condition assessment model and scale for subway stations.