This paper describes the use of fluid curtains for the mitigation of problem clouds. Mechanical and physico-chemical actions of spray barriers are examined. Practical engineering codes are developed with the support of experimental findings gathered on dedicated facilities. It is shown that forced dispersion is more appropriate for the dilution of flammable gases, while the efficient dispersion of a toxic dense pollutant requires the use of inhibiting liquid curtains. The design rules of these two types of spray curtains exhibit important differences. Mechanical curtains require coarse upward sprays in combination with a small upstream fence. Physico-chemical absorbing barriers rely on fine sprays; therefore, additional studies of wind effect are needed. The concept of the dual diluting/inhibiting curtain is suggested for future investigation.