Didemnum vexillum Kott (2002) is a high-impact, globally-invasive, colonial tunicate species that is native to Japan (Lambert, 2009; Stefaniak et al., 2012). It is generally a temperate cold-water organism, and its introduced range currently includes New Zealand, the Netherlands, France, Ireland, United Kingdom, Spain, Italy, and both the west and east coasts of the United States and Canada (Lambert, 2009; Stefaniak et al., 2012; Tagliapietra et al., 2012; Ordóñez et al., 2015, Vercaemer et al., 2015). Like other invasive tunicates, D. vexillum has the capacity to reproduce rapidly, outcompete native species, deteriorate environmental integrity, and cause significant economic harm (Lambert, 2005; Blum et al., 2007; Daniel and Therriault, 2007; Langyel et al., 2009; Cordell et al., 2013). For these reasons, this document aims to increase awareness of D. vexillum, with a focus on identification, natural history, current global distribution, potential impacts, and prospects for management and control where introductions occur.