Onium-poly hydrogen fluorides, the amine or ether complexes of hydrogen fluoride, have been widely used as fluorinating agents in organic synthesis. A variety of fluorination reactions involving electrophilic additions to unsaturated compounds, nucleophilic substitutions, desulfurative fluorinations, strained ring-opening fluorinations, and deprotection of acetal, ketal and O-silyl ethers could be achieved with these onium-poly hydrogen fluorides. Fluorination of alcohols could be achieved under either SN1 or SN2 conditions, depending on the reagent choice and reaction conditions. Deaminative fluorinations using these reagents provide convenient access to chiral α-fluoro acids. As strong Bronsted acids they can be used to catalyze the electrophilic alkylation of alkanes with alkenes to give high-octane gasoline-range hydrocarbons. They also serve as high dielectric constant ionic liquid media. In addition, polymer-supported onium polyhydrogen fluorides such as polyvinylpyridinium polyhydrogen fluoride (PVPHF) have been developed as convenient alternative reagents, which could be recovered and recycled for further use.