Additives can be used to enhance the biological performance of glyphosate, as well to improve the physical characteristics of its prediluted formulations. Their effects on efficacy can be mapped through a series of consecutive processes that the herbicide has to follow from spray tank to ultimate biological site of action. Key factors affecting dose transfer are discussed, including retention on foliage in the glasshouse and field, the effects of adjuvants on foliar uptake and the relationship between potential adjuvant effects and glyphosate concentration. A better understanding of the relative importance of the main barriers to glyphosate activity and the ability of adjuvants to overcome them should enable a more rational approach to the design of glyphosate formulations in the future.