Many animals and plants now occur in places well beyond their native ranges, as ‘alien species’. Whether as natural dispersers, accidental imports or deliberate introductions, many species actively invade and colonise their new areas and in more extreme cases become ‘naturalised’ as accepted permanent occupants. Impacts on the native biota of colonised areas can be severe, and many extinctions of endemic insects have been attributed to their being displaced or outcompeted by alien taxa. Their study can also provide substantial basic knowledge useful in conservation.