Many major accidents or failures have occurred in long established organisations with well developed safety management systems. Recent examples include Buncefield, and BP Texas City and Deepwater Horizon. Such incidents have underpinned the notions of drift and organisational amnesia. Research into the causes of drift and organisational amnesia identifies two pre-conditions, being: degradation of key aspects of operational standards (such as competencies and safety awareness); failure of oversight processes to detect and reverse the degradation. The presentation draws together a model of drift and organisational memory, and potential defences, with examples highlighting the lessons learnt for operators of complex major hazard facilities.