The invasive kelp Undaria pinnatifda has a global non-native range and is considered one of the "world's worst invasive species". It has been present in the UK for at least 25 years; however given its status, there remains a dearth of information regarding its ecology, impacts and management feasibility. A variety of surveys and manipulative techniques were implemented in rocky reefs and marinas of the southwest UK to better support the design and prioritisation of management, and to advance ecological understanding of marine invasive species more generally. This thesis consistently highlighted that marinas are paramount to the successful spread, proliferation and reproductive fitness of Undaria. Excluding or limiting its abundance in marinas may, therefore, restrict the likelihood of its spread to new regions and its proliferation to surrounding natural habitats. Management feasibility, however, was considered to be low, due to targeted removal measures in marinas altering recruitment patterns and even promoting total recruitment, and the high inter-habitat and inter-annual variation recorded in Undaria population dynamics. Within natural rocky reef habitats, Undaria was absent or found in low abundance in areas of high wave exposure, high desiccation stress, and where there was high abundance of the native perennial Laminaria spp. Undaria is therefore likely to be restricted in the potential range into which it can proliferate. Where Undaria had invaded rocky reef communities, there was a consistent and significant impact upon the native annual canopy-forming macroalgae S. polyschides. The overall ecosystem impact of Undaria on rocky reef communities of the UK, however, is likely to be small, with no consistent impacts identified for any other macroalgal species, including the canopy dominant Laminaria spp. There are cases where targeted management of Undaria may be proportionate and feasible; however, in many locations around the UK, Undaria is likely to remain unmanaged and will become an accepted part of the biota.