It is well recognised that pests and pathogens are increasingly threatening both natural woody ecosystems and plantation forestry world-wide. This threat is largely connected to the increasing movement of people and products globally, with concomitant introductions of pests and pathogens into new environments. Typically, these invasive alien organisms are accidentally transferred from areas where they are native to the same or closely related tree hosts, to elsewhere in the world. For tree pathogens, there are many well known examples such as those for the causal agents of Dutch elm disease, chestnut blight, white pine blister rust and pine wilt caused by the pine wood nematode. Equally well known examples for insect pests are the gypsy moth, the Asian longhorn beetle, various conifer infesting bark beetles and the emerald ash borer. In addition to these pests and diseases, the introduction of which should be easier to predict given knowledge of patterns of introduction, there is growing evidence that novel host, pest and pathogen interactions are evolving. In this situation, host shifts are apparently occurring where pests and pathogens are able to infest or infect trees that would previously have been considered as non-hosts. There are also intriguing, yet very worrying, new associations emerging between insects and pathogens that are able to cause substantially greater damage to the host trees than has been known for these organisms in the past. In general, these novel associations are poorly understood and due to their unpredictable nature, they seriously complicate quarantine efforts. There is clearly an urgent need to gain knowledge regarding patterns and processes underlying the emergence of host shifts as well as novel pest and pathogen interactions. Research regarding novel associations and host shifts, which might in the past have been considered esoteric, should clearly gain vigorous support in order to reduce an emerging new threat to global forests and forestry.