The large number of pesticides which are frequently required for crop protection makes it impracticable to apply them individually to crops, and there has been increasing resort to the use of mixtures of chemicals. This presents difficulties, however, in that the overall effect of a mixture is often different from the sum of the effects of its components: this may be apparent as reduced performance or even crop damage. This article describes some of the most severe antagonisms that have been observed, the results of investigations into their causes, and methods of overcoming them.