Hawkweeds, from the Pilosella and Hieracium genera, are significant invaders of cool climate grasslands and herbfields, reducing productivity and impacting on ecosystem processes and species richness. With significant research on the ecology and biology of hawkweeds, this review identifies a number of key characteristics that influence invasion into native habitats. The ability of hawkweeds from the Pilosella genus to spread vegetatively, through rapidly-growing stolons that produce daughter rosettes, and to produce apomictic seeds makes species in this group significant invaders, particularly P. officinarum. Rapid colonisation of disturbed sites has been observed in many continents where agriculture or other anthropogenic disturbances are present. Rapid increases in colonisation appear more obvious over the last 50��years perhaps associated with time of arrival and/or increased grazing and pasture improvement of native grasslands enhancing colonisation. The production of apomictic seeds in all species provides a rapid pathway of invasion at early stages of establishment. Hybridisation amongst species may hamper the development of biological control agents in the future.