Of malignant tumours with a propensity to invade the perineural space, adenoid cystic carcinoma of the salivary glands is perhaps the best known. However, it is not known if microscopic evidence of perineural invasion has true prognostic significance in adenoid cystic carcinoma. A review of the available data, which is the aim of this article, reveals the issue is not straightforward. There is a plethora of conflicting data which, on balance, suggest the answer is indeed in the affirmative, particularly if the nerve involved is above a certain size, or "named". Even if the data are equivocal, many head and neck surgeons and oncologists take account of histologically proved perineural invasion when planning treatment for adenoid cystic carcinoma.