Biological invasion has become a serious ecological and economic problem. Invasive pests occupy the niche in native species, reduce biodiversity in introduced range, affect the structure and function of ecosystem, and cause great economic losses on agriculture, forestry and stockbreeding. Feasible methods to control noxious invasive species are needed urgently. Biocontrol is one of the most important methods for controlling invasive pest. With this method invasive pest could be controlled economically, effectively and persistently without harmful effects on environment. Traditional biocontrol has become one of the most important methods for controlling noxious alien invasive weeds. In this paper, a brief introduction to biocontrol was given firstly. Then the theoretical basis for traditional biocontrol, i.e. enemy release hypothesis, and the direct and indirect effects of biocontrol agents on target alien invasive species and on nontarget native species were detailed. Enemy release hypothesis is one of the oldest and most influential hypotheses. It attributes the success to the fact that exotic species, upon introduction, are liberated from their specialist herbivores and pathogens. Release from strong suppression by enemies in their native range enables exotic species to attain higher densities and wider distribution in their introduced range. To test the hypothesis, many field surveys, common garden experiments and enemy exclusion experiments have been conducted on biogeographical levels and community levels. However, the hypothesis is not confirmed on some species, indicating that the success of these species can not be attributed to enemy release in their introduced range. For this kind species, biocontrol may not attain expectative results. Biocontrol should be used to control the species whose success is due to enemy release in their introduced range. Biocontrol agents can influence exotic species fitness and competition, reduce their abundance and distribution in their introduced range. However, the potential risk of biocontrol must be kept in mind in practicei; biocontrol agents may induce direct effects through host shifting and indirect effects on nontarget native species through ecological replacement, compensatory responses and food-web subsidies. The potential risk can be reduced or even eliminated through using efficacy and host-specific biocontrol agents. Therefore, it is very important in practice to test the efficacy and host-specific of biocontrol agents. Finally some suggestions of biocontrol in China were given.