Jets can occur as a result of flow under, through or over hydraulic structures. Several forms of jets are including plunging jets, submerged jets, horizontal and vertical jets and dimensional jets. The chapter deals with the flow characteristics of jets and of the surrounding mass. Important design parameters with respect to jet scour are the equilibrium scour depth and the length and width of the scour hole. It discusses the dimensions of the scour hole for some types of jets. Jets can occur as a result of flow under, through or over hydraulic structures. The scour associated with hydraulic structures may be caused by different types of jets, including plunging jets that impinge on an erodible bed and horizontal jets eroding bed material immediately downstream of structures. The relatively high velocities in jet flow, together with the high turbulence level, result in scouring of the river.