Cracks are the most dangerous defects of a welded joint. During fusion welding, the resulting welds may have either cold or hot cracks. Hot cracks are normally located in the weld metal and formed during solidification or at a temperature below the solidus temperature for a given metal. As the polygonization and cold cracks are formed in the solid metal, the effect of surface phenomena on the processes of their formation shows up to a much lesser degree than on the formation of solidification cracks. The process of formation of solidification cracks in the weld is affected to a lesser but still marked degree by the silicon content of the metal. The critical carbon and silicon contents of metal also depend on the shape of the weld pool. Various methods for estimation of susceptibility of welded joints to solidification cracking, allowing for the effect of the base metal, filler metals, and the type of the welded structure, are available.