Possible mechanisms of sympatric speciation in insects were discussed in view of their biology, ecology and genetics. Disruptive selection on sympatrically incipient species, which is associated with distinctive divergence on ecological and genetic traits, promoted complete or partial reproductive isolation between sympatrically related populations. Character displacement, assortative mating and sexual selection were involved in the course of sympatric speciation. Host_specific insects were good cases for interpreting sympatric differentiation and most of them were phytophagous. Verifiable criteria for insect sympatric speciation proposed by some evolutionists and entomologists mainly encompassed the test of assortative mating, quantification of gene flow, ascertaining the extent of genetic differentiation, estimation of linkage disequilibrium among loci and hybrids fitness, at the like It was possible to demonstrate sympatric speciation in some insects, but further research was needed to elucidate the mechanisms of reproductive isolation.