This document discusses possible explanations for the Great Vowel Shift, a major phonological change in English pronunciation that occurred between 1400-1700. It describes the vowels and diphthongs that were affected and outlines Otto Jespersen's eight step model of the shift. Several potential causes are then presented, including the English Reformation, the Norman Conquest, the Black Death, drag and push chain theories, and social factors like the rise of the London middle class. While the exact cause remains unknown, most scholars argue the shift arose from the need for a common dialect to facilitate trade and commerce in London. The implications of the Great Vowel Shift on English spelling and reading are also noted.