Electronic materials play an important role in advancing the semiconductor and microelectronic packaging technologies. This chapter will give an overview of major electronic packaging materials–flip‐chip underfills. Fundamental knowledge about underfill, including history, process, types, compositions, and properties, was described. Underfill filler technology for fine pitch application was explained in terms of surface treatment mechanisms and underfill material properties. Advanced underfill technologies for mobile device application, wafer‐level application, and through silicon via (TSV) three‐dimensional (3D)‐IC were also introduced.