There are a lot of insects and mite pests in hazelnut orchards in Turkey. Most pests affect hazelnut production through direct or indirect injury, but a few affect kernel quality and reduce yields by feeding on the nuts themselves. In recent years, hazelnut exporters have begun to complain about this kind of damage reducing kernel quality because it has a negative effect on exports. It was established that more than 15 bug species could affect kernel quality (Heteroptera: Pentatomidae, Coreidae and Acanthosomatidae) in Turkish hazelnut orchards. This study determined fluctuations in the population of these bugs in six hazelnut orchards and on three main cultivars. Palomena prasina and Gonocerus acuteangulatus were found to be the main species. The pest population was above the economic damage threshold. In a cage experiment with P prasina, three types of injury were determined. In addition to field and cage experiments, quite significant data about bug-related kernel damage were obtained from hazelnut factories. Kernel damage varied according to variety, locality and sample, reaching levels of around 20% of the total crop.