Summary.Published literature on diquat (9,10‐dihydro‐8a, 10a‐diazonia phenanthrene dibromide; 1,1′‐ethylene‐2,2′‐dipyridylium dibromide) and related dipyridyl herbicides is reviewed. Their activity is probably connected with the formation of stable free radicals on reduction and both photosynthetic and respiratory processes may be associated with free radical formation in the plants. These herbicides are rapid‐acting translocated desiccants and kill the aerial parts of most plants, but are safe as soon as they touch the ground, having no residual effect through the soil. Possible uses are as desiccants, in place of cultivation to clear fallows and seedbeds before sowing, in pre‐emergence weedkilling, for weeding round trees and bushes and, with directed spraying, for inter‐row weeding in row crops.Revue des essais effectués avec le Diquat et