The background and the basis for conducting a Technical Due Diligence are discussed as well as the source information, typical studies and potential limitations. Typical short comings including show-stoppers' are presented for the metallurgical area, with examples. Issues found in other areas such as geology, mining, infrastructure and financial evaluations are briefly reviewed, highlighting the input and interaction required by metallurgists with these disciplines. A checklist is provided as a guide to maximise the success of a Due Diligence as well as a check on the quality of a Feasibility Study.CITATION:Newell, A J H, 2015. Fatal flaws in technical due diligences, in Proceedings MetPlant 2015, pp 61-77 (The Australasian Institute of Mining and Metallurgy: Melbourne).