Energy security is a key driver of energy policies in most countries. Security of electricity supply is increasingly considered a central part of energy security. This study focuses on two key areas of electricity security: diversity and aging of power plants. It is an analysis with global coverage with aggregation of data at the level of individual countries. This thesis uses the more advanced ‘triple concept’ approach to measuring diversity of fuels in the power sector which includes the ‘distance’ between the types of fuels as well as the number of types and the evenness of spread between them. It also analyses the physical diversity of power plants. This study also assesses the aging of power plants in a new and novel way, using a specifically developed Aging Index. The study then assesses which countries are vulnerable under each of the areas of analysis.