Natural foods, despite their high water content, are made solid by the conning water within the cells. In contrast, fabricated foods with solid-like (visco-elastic) properties are almost always hydrogels (water conned in a polymer matrix). Most gelling carbohydrates and gelatin (protein) form hydrogels when their concentrated solutions are cooled; subsequently, however, these melt upon heating (i.e., are thermo-reversible). Surimi, on the other hand, like the muscle proteins of other animal species, as well as egg white, wheat gluten, and milk b-lactoglobulin, forms thermo-irreversible gels upon heating, which do not melt with further temperature change. Furthermore, surimi is known to produce gels of very high strength and deformability. It is the excellent heat-induced gelation properties of surimi that make it useful as a food ingredient. This chapter will review the chemistry of muscle proteins with regard to surimi gelation, as affected by various factors associated with the manufacturing of surimi and surimi seafoods.