Critical infrastructures (CIs) are those physical and virtual systems or aggregation of assets that provide essential functions and services that support societal, economic and environmental systems. ). These systems are regarded as “lifeline” or “critical” infrastructures, with the services they provide deemed vital for day to day operations, economy, security, healthy and overall wellbeing of modern societies. We recognise these infrastructures as the power we use in our homes and offices, the water we drink, the different transportation systems that we use to move from place to place, the communication systems that we depend on to stay in contact with family and friends and which we depend upon to conduct business, as well as the police that keeps us safe and the emergency services we call on when we spot a fire.
In the European Union (EU) CI is defined as:
 Council Directive (EC) 2008/114/EC of 8 December 2008 on the identification of European critical infrastructures and the assessment of the need to improve their protection.
An asset, system or part thereof located in member states which is essential for the maintenance of vital societal functions, health, safety, security, economic or social well-being of people, and the disruption or destruction of which would have significant impact in a member state as a result of the failure to maintain those functions
An indicative list of 11 CI sectors have been outlined by the European Commission in a green paper on a European Programme for Critical Infrastructure Protection . These are:
 Commission of the European Communities (2005), Green Paper on a European Programme for Critical Infrastructure Protection. COM (2005) 576 Final.
Systems that fall under such CI sectors will be regarded as CIs in the RESILENS project.