Thanks to technological advancements, critical infrastructures integrate many smart technologies and become highly connected to the cyber world. This is especially true for Cyber-Physical Systems (CPSs), which combine hardware and software components. Despite the advantages of smart infrastructures, they remain vulnerable to cyber threats and adversarial events such as cyber-attacks. This talk focuses on quantifying the cyber resilience of complex systems modeled with a multi-layering approach. As a use case, we consider the Secure Water Treatment System (SWaT) testbed.