Ecology is the study of the relationships of communities of living organisms with their environments and each other, in particular the factors affecting their numbers. Pest control is, essentially, modification by man of these numbers in an agricultural environment, and it is argued that investigations of and a general awareness concerning ecological factors are essential prerequisites for proper effectiveness. Examples from Malaysian plantations are given of major outbreaks and great expense arising from failure to observe these principles and, on the other hand, of ecological studies forming the basis of successful and cheap control procedures. Plantations exhibit a high degree of biological stability, especially in the continually warm and humid climate of Malaysia, so that many pests are of only potential severity, breaking out spasmodically, but conditions are also ideal for the existence of pests for their rapid increase if this stability is upset. Malaysia is predominantly agricultural and much dependent on efficient plantation production. Pests menace efficiency, but the insurance of continually increasing ecological knowledge will keep to a minimum both the economic impact of pests and the considerable risk of severely aggravating pest problems by unsuitable control measures. Some consideration is given to relating pest control practice in Malaysian plantations with modern thinking on environmental problems.