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Tasmanian Herbarium (HO) collections are shown to provide temporal and distributional data to monitor weed introductions into the State flora during the last 25 years. Information obtained from herbarium collections, the botanical literature, and anecdotal sources indicates that since 1970, 159 new plant taxa have been recorded as naturalised in Tasmania, bringing to a total over 740 weed species recorded from the State. Most of these species are from the families Poaceae (15.1%), Fabaceae (10.1%), Asteraceae (6.9%), Cyperaceae (5.7%), Rosaceae s.l. (5.0%), Caryophyllaceae and Liliaceae s.l. (3.8% each), Iridaceae (3.1%), and Juncaceae and Ranunculaceae (2.5% each). While for many taxa the mechanisms for introduction remain unknown, at least 35% were introduced as ornamentals, and some 5% arrived through agricultural practices. Of the 159 species, 19 are known only from Tasmania and have not been recorded from the Australian mainland. The majority of weeds are of European origin, with a high proportion being from Africa, North and South America, and mainland Australia, in that order. This study demonstrates that even with the current quarantine controls a large number of weed species have been introduced to the State flora in the last 25 years, and a considerable number of these species are recognised as potential environmental weeds. The number of new weed species recognised is also possibly due, in part, to more collections of weeds being undertaken in recent years. As a large percentage of the weeds identified are ornamentals, stricter controls on the introductions of new ornamentals may be needed. Some of the limitations of using herbarium collections to assess weed introductions are also discussed.