Pesticides are commonly applied either as aqueous solutions or emulsions, using spray tips designed to break up the liquid into a relatively fine spray. One consequence of this practice is that a variable proportion of the spray, composed of smaller droplets, drifts out of the target area. Some weedkillers, such as the derivatives of 2,4-D, are effective in minute quantities and a small amount of drifting spray can, therefore, cause considerable damage to sensitive crops grown down-wind. If the spray contains an insecticide the same amount of drift will take place under the same conditions. The insecticide is not likely to damage adjoining crops but it may not be wanted, it may cause undesired contamination of pasture land, it may be harmful to domestic animals or to wild life, and it can be said to contribute to the much-discussed problem of environmental contamination. However, it is not the purpose, here, to pursue the controversy about the use and misuse of pesticides. Sufficient to say that drift always results in some wastage of valuable spray liquid, it detracts from the efficiency of spraying operations and it is a problem which everyone would like to overcome.