This article examines the differences and commonalities between the concepts ‘Protection of Civilians (PoC) and ‘Responsibility to Protect’ (R2P) in terms of their origins, evolution and applicability to various situations. Such comparative analysis is necessary as to avoid confusion and misinterpretation. The main argument is that the two can be regarded as ‘sister’ concepts, reinforcing each other, particularly when it comes to critical situations, the most recent example being the international responses to the deadly threats to civilians in Libya in February-March 2011 and the measures imposed by the UN Security Council Resolutions 1970 and 1973. The article also assesses whether the responses to the crisis in Libya represent a triumph or a failure of the ’sister’ concepts.