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Silica is absorbed from the soil by many grasses in an active or passive manner depending upon the species involved. It is carried upwards in the transpiration stream and deposited throughout the plant where it polymerizes to form amorphous silica gel. Deposition appears to be a passive process but some active mechanism may be involved. Deposition follows a set sequence and may be initiated before leaf expansion. Silica benefits most plants but may have adverse effects upon herbivores ingesting this plant silica. A number of possible roles have been suggested for silica deposition in plants such as the ?light window? hypothesis or as a defence against large herbivores and these are discussed in the light of the available evidence. It is concluded that leaf silicification evolved in response to invertebrate herbivory and is primarily aimed at reducing tissue loss to these herbivores.