The synthetic applications of isoureas as alkylating agents are evaluated and reviewed. Esterifications employing isoureas were reported many years ago, but extended applications were inhibited by lack of isourea availability. A high-yield, mild synthesis of a major group of isoureas first appeared in 1955. This spurred numerous exploratory efforts which have extended alkylation reactions to a variety of carbon- and heteroatom-containing groups. These procedures are discussed in detail with respect to generality and selectivity. A discussion of the probable mechanism helps explain the observed selectivity and indicates potential developments of isoureas as general alkylating agents. A brief survey of the large number of available isoureas supports predictions as to their importance as reagents for analysis and organic synthesis. 1 . Introduction 2. Carboxylic Ester Formation 3. Phosphate Ester and Phenyl Ether Synthesis 4. Sulfur Alkylation 5. Nitrogen Alkylations 6. Active Carbon Alkylations 7. Isourea Rearrangements/Internal Alkylation 8.1 Mechanism of Alkylation 8.2 Heteroatom Selectivity in Alkylations 9. lsourea Availability