Abstract Ab? Yazid al?Bist?mi (d. 874 AD) was a renowned early s?fi who exerted a tremendous influence upon the doctrinal formulation of the sufism of medieval times. A highly controversial figure, he is venerated by some as a top?ranking saint and s?fi, condemned by others as a notorious heretic, and there are still others who suspend judgement on him. More than 200 years after him al?Ghaz?li (1058?1111 AD) flourished as the greatest s?fi of all times; he examined and evaluated the (...) teachings of his s?fi predecessors including Ab? Yazid. To determine his evaluation of Ab? Yazid and his opinion on the related, well?known concept of man's union with God at the highest peak of spirituality is the main aim of this paper. To achieve this aim al?Ghaz?li's citations from Ab? Yazid's teachings on many basic doctrines of sufism, together with his explicit comments on them, are analysed in the second section of the paper, and he is found to have evaluated these teachings as of a very high grade and to have extolled Ab? Yazid as a s?fi of the highest rank. The third section studies al?Ghaz?li's opinion on the most important aspect of Ab? Yazid's teachings, i.e. his shatah?t or ecstatic utterances apparently expressive of union, fusion and divine indwelling. This began with a consideration of al?Ghaz?li's definition of two kinds of shath and his condemnation of them on the grounds of their harmful consequences. In connection with a study of his condemnation of the shatah?t of Ab? Yazid and al?Hall?j an investigation is made into his opinion on union and fusion. It is found that throughout his s?fi life he condemned them as false concepts. However Ab? Yazid's shatah?t, which apparently mean union, fusion, etc. are interpreted in an orthodox manner, and he is adjudged an elect of the elect, a gnostic who reached the level of reality of realities, a perfect s?fi who attained to God. All the above findings are based on al?Ghaz?li's explicit comments on Ab? Yazid. The fourth section of the paper deals with his implicit, indirect comments which also prove his appreciation of, and indebtedness to, Ab? Yazid in respect of several central concepts of sufism. (shrink)