The reactor pressure vessels (RPV) is central to a nuclear reactor, and is therefore a life-determining structural component, i.e. RPV damage left unaddressed could jeopardise the continuing operation of the power plant. The main degradation mechanism affecting RPVs is irradiation embrittlement, where neutron bombardment from the nuclear reaction displaces atoms in the RPV steels. Inspection, testing, and appropriate maintenance are therefore required to ensure any damage is mitigated and thereby to allow the plant to continue generating power. This book critically reviews the irradiation embrittlement as it affects RPV steels, along with mitigation routes to manage the RPV lifetime.