Publisher Summary This chapter discusses that the glycosyl halides are compounds in which the hydroxyl group at the reducing carbon atom of a sugar is replaced by halogen. As such, the glycosyl halides are rare, only a few fluorides being well authenticated. However, the fully acylated glycosyl halides, particularly the acetylated compounds, sometimes referred to as “acetohalogeno sugars” or “acetohalogenoses”, are very well known. The most familiar of these is 2,3,4,6-tetra-O-acetyl-α-D-glucosyl bromide (“acetobromo-α-D-glucose”). The poly-O-acyl derivatives of the glycosyl halides are among the most important intermediates for synthesis in carbohydrate chemistry; moreover, their chemistry has considerable intrinsic interest. Most of the methods for the preparation of glycosyl halide derivatives depend on the replacement by halogen of an acyloxyl (nearly always acetoxyl) group a t the reducing carbon atom. Hence, the methods for the synthesis of a large number of the glycosyl halide derivatives can be exemplified by description of those used for converting fully acetylated sugars into O-acetylglycosyl halides. As might be expected, treatment for prolonged periods often results in replacement of a second acetoxyl group. Other methods available for the synthesis of glycosyl halide derivatives involve the replacement of hydroxyl and alkoxyl by halogen, the opening of anhydro rings with hydrogen halides or equivalent reagents, and the addition of halogen or hydrogen halide to certain unsaturated sugar derivatives. Glycosyl iodide derivatives are usually prepared from the corresponding bromides by reaction with sodium iodide.