Despite the great importance of chemical insecticides, with the ever increasing resistance of pest insects to chemical insecticides and the growing concern over environmental pollution, it becomes evident that the problem of pest attack on crops cannot be solved by anyone system. Under these circumstances, main pathogens of insects, i.e., viruses, bacteria, fungi and protozoa, have been studied to control many insect pests. Some of these pathogens are now being produced as microbial insecticides at the rate of hundreds of tons per year in the world. Some microbial insecticides are very effective against numerous or target insects under suitable environmental condition, and microbial control has been played an important role in integrated control program. They have many unique properties such as selectivity, multiplication and harmless to higher animals. However, we must be aware also that there are many problems to be solved, such as safety, persistence and difficulty of efficacy, etc_ on the microbial insecticides.