The Southern Ocean is a critical component of the global climate system and an important ecoregion that contains a diverse range of interdependent flora and fauna. The Southern Ocean also hosts numerous fronts: sharp boundaries between waters with different characteristics. As they strongly influence exchanges between the ocean, atmosphere and cryosphere, fronts are of fundamental importance to the climate system. However, rapid advances in physical oceanography over the past 20 years have challenged previous definitions of fronts and their response to anthropogenic climate change. Here, we review the implications of this recent research for the study of climate, ecology and biology in the Southern Ocean. We include a frontal definition "user guide" to clarify the current debate and facilitate future research. The Southern Ocean, generally defined as the global ocean south of about 35 • S that encircles the Antarctic continent, is unique oceanographic environment due to the lack of continental barriers blocking its flow and the strong winds that blow over its surface 1. At large scales, the Southern Ocean is characterised by both the intense eastward flowing Antarctic Circumpolar Current (ACC), one of the most powerful current systems on Earth, and strongly tilted isopycnals (lines of constant density) that shallow to the south. Observations of the Southern Ocean dating back to the Discovery expedition in the 1920s revealed that the transition from warmer subtropical waters to colder Antarctic waters does not occur smoothly, but is concentrated into a series of sharp transition zones, aligned generally east-west, that have come to be called fronts 2. Further observations revealed that salinity, oxygen, nutrients and various other tracers showed similar behaviour, and that between the fronts, water properties are relatively homogeneous. As such, fronts delimit the boundaries between different water-masses with distinct environmental characteristics 3. These fronts also tend to coincide with the location of narrow yet very intense currents known as "jets" 4 that dominate the ACC's flow 5. The Southern Ocean is divided by fronts into a number of distinct biophysical zones, and hence a number of distinct habitats, which in turn support distinct biota 6, 7. Numerous studies have shown that seabirds and marine mammals tend to congregate and forage in and around fronts 7. As the Earth continues to warm due to anthropogenic climate change, it is vital that we understand how these fronts and jets will respond to changes in the global climate system, and what influence that might have on associated ecosystems 8-10. Due to its remoteness and harsh climate, undertaking field studies in the Southern Ocean is both difficult and expensive. As a result, the Southern Ocean is amongst the most data-sparse of all major ocean basins, which has hindered progress on key questions regarding its dynamics and ecological communities 10. In recent decades however, a deluge of new data from satellites and Argo profiling floats, along with e...