This article investigates the Middle English (ME) spelling evidence for the development of early Middle English /u:/, which diphthongised in the so-called ‘Great Vowel Shift’. After 1066, the Anglo-Norman spellings <ou> and <ow> were gradually adopted instead of <u>, and as these digraphs may equally well correspond to diphthongised reflexes as to monophthongal ones, certain evidence for diphthongisation is hard to come by, especially for the early stages. Fifteenth-century and later <au> and <aw> spellings are clear indications of advanced diphthongisation. This article makes use of the spellings contained in LALME and LAEME, two linguistic atlases of localised Middle English material, examining the claim that the vowel shift was much earlier than has been assumed. When early aberrant spellings are examined in the light of the entire orthographic systems in which they appear, it becomes clear which spellings may be used to support the early vowel shift hypothesis, and which must be discarded.