When faced with not being able to find a "difficult parathroid", the keys to successful completion of the procedure at the time, or successful re-operation, are : 1. review the localisation studies, specifically the images rather than the report. For example nearly 50% of abnormalities reported as a “lower parathyroid adenoma” turn out to be a descended upper parathyroid lying in a para-oesophageal location on the pre-vertebral fascia. This is the most common cause of a “missing” adenoma as the surgical approach to the pre-vertebral fascia is very different to dissection of the lower pole/thyrothymic region. 2. develop an “embryological based game plan” for further surgical exploration. This depends on assessing which normal glands have already been identified and aligning the “missing” gland with associated branchial cleft structures. For example Parathyroid III is associated with the thymus and the vagus nerve, so a missing lower parathyroid gland leads to an initial search of the lower lobe of the thyroid, the thymus, and the carotid sheath down to the sternal notch. Parathyroid IV is associated with the internal carotid artery and neural crest elements including C-cells, so a missing upper parathyroid gland leads to a search around the posterior thyroid including Tubercle of Zuckerkandl, pre-vertebral fascia, and upper half of the carotid sheath. Embryological symmetry is also important with contralateral parathyroid location frequently a clue to the missing gland, eg a normal intrathymic parathyroid on one side should lead to a thorough thymectomy, including the upper sternal component, on the contralateral side.