Critical infrastructures and entities protection from threats and hazards has become increasingly critical in modern society, which is more and more dependent on supplied services. The importance of the topic has been proved by the interest of the European Union in developing a common policy addressing critical entities protection. This chapter aims to introduce the reader into the resilience of critical entities, which is a key concept in critical entities protection. The first section depicts the scenario of critical infrastructures/entities, illustrates the reasons that make them worthy of protection, gives some clues to traditional protection approaches and related limitations. The second section presents a conceptual model of resilience and its analysis dimensions, while the third paragraph illustrates the resilience indicators and the related assessment framework; the final paragraph, preceding the conclusions, consists of a brief excursus on European Union policies on Critical Infrastructure protection.