Impact of invasive species, along with direct habitat loss, is considered to be the biggest cause of biodiversity loss. One of the most common routes of transmission of invasive plant species is for horticultural purposes. In the middle of the 19th century, Reynoutria japonica Houtt. and Reynoutria sachalinensis (F. Schmidt) Nakai were introduced to Europe. Soon they started to hybridize and to produce Reynoutria × bohemica Chrtek et Chrtková. All three species are collectively called invasive knotweeds. Invasive knotweeds are characterized by their exceptional ability to regenerate from all parts of the plants. That ability have earned R. japonica place on the IUCN list of the 100 worst invasive species in the world. Although the hybrid R. × bohemica deserves a place on the list, it is not on it due to the relatively late description of species and a century-old misidentification. Therefore, it is widely believed that R. japonica is the most invasive species of invasive knotweeds. Currently, invasive knotweeds are spread over five continents, while R. × bohemica is the most common species worldwide and in Croatia. Although the actual distribution of hybrids is probably higher. This paper describes the characteristic morphology by which species can be identified. According to the classification of invasive alien species (Blackburn et al. 2014), invasive knotweeds belong to species with high environmental impact. However, impact could be underestimated due to lack of systematic research of impacts on vertebrate and river and stream ecology. Invasive knotweeds are extremely difficult and expensive to manage, so the way we deal with them should be changed. It is suggested that management practices should focus on prevention of further spread. Also, due to the possibility of interspecific hybridization and the creation of a potentially more invasive hybrid, it is necessary to prevent formation of seeds.