Forest pests are a major threat to forest ecological and economic health. For this reason, the monitoring for forest pests was necessary and the monitoring program has begun from 1968 in Korea. During past 50 years, forest pests in pine forests have been shifting from pine moth (Dendrolimus spectabilis) via pine needle gall midge (PNGM, Thecodiplosis japonensis) to pine wilt disease (PWD). The pine moth is a native species to Korea and its outbreaks were recorded even 500 years ago. It was the major forest pests in Korea from 1950s to 1970s. The cause of its decline was probably due to increase in activities of microorganism-type natural enemy. In 1980s and 1990s, PNGM was the major forest pests in Korea. It invaded to Korea in 1929 and dispersed to nationwide in 1992. The suite of parasitic wasps to PNGM at least partially contributed to decline of PNGM on the basis of long term monitoring research. After 2000s, the pine wilt disease caused by Bursaphelenchus xylophilus, an invasive species and vectored by Monochamus alternatus and M. saltuarius, native species was the major forest pest in Korea. These results showed that threat by invasive species on the forest health was increase and natural enemies at least partially contribute to stabilize outbreak of forest pests. Therefore, the further researches and monitoring on invasive species and roles of natural enemies are intensively conducted.