Proteinase inhibitors (PIs) are anti-metabolic proteins, which interfere with the digestive process of insects. It is one of the important defence strategies existing in plants against predators. The use of the plant-derived PI genes for developing insect-resistant transgenic crops has come of age. Several transgenic plants expressing PIs have been created and these plants have shown enhanced resistance against insect pests. Recently PIs have also been used to engineer resistance against viruses in transgenic plants. The current scenario for developing insect, fungal and viral pathogen- resistant transgenic crops by using PI gene(s) is reviewed. The biosafety aspect of the transgenic plants expressing PIs is also discussed.