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An adjuvant is a chemical that can be added to a pesticide spray mixture to enhance and modify the physical properties of the pesticide for the spray mix. An adjuvant is a specialized chemical that must be matched to a particular pesticide, the problem, and environmental conditions to ensure that they enhance and do not detract from the effectiveness of the pesticide.

Based on their intended use or type of action, adjuvants can be divided into three broad categories: activators, spray modifiers, or utility modifiers.

• Activators – increase the biological activity of the pesticide beyond that usually obtained without the material added. This class of adjuvant is particularly useful when applying herbicides. There are five major classes of activator adjuvants: 
  • Surfactants
  • Vegetable oils 
  • Crop oils 
  • Crop oil concentrates 
  • Methylated seed oils.
• Spray modifiers – modify the spray solution in the tank on the way to the target itself. There are four major classes of spray modifiers: 
  • Spreaders
  • Stickers
  • Foaming agents
  • Thickening agents.
• Utility modifiers – widen the range of conditions under which a given pesticide formulation is useful. There are four major classes of utility modifiers: 
  • Antifoam agents
  • Compatibility agents
  • Buffering agents
  • Water conditioning agents

Derived from the words surface-acting agents, a surfactant is a compound that improves the emulsifying, dispersing, spreading, wetting, or other surface-modifying properties of liquids. Whereas surfactants make up a large class of adjuvants, not all adjuvants are surfactants.