Simply put, phylogeography is a sub-discipline and extension of biogeography that looks at the distribution of taxa and their various degrees of relatedness simultaneously. One of the hallmarks of phylogeography as a discipline that differentiates it from biogeography is that the latter concentrates on patterns of distribution exclusively whereas the former also takes into account the pro- cesses that were involved in the creation of these patterns (Avise 2000). While many studies on biogeography will also discuss the implication of their ndings for our understanding of the processes involved (Clarke 2008), phylogeogra- phy does this more explicitly so by making the evolutionary history of the taxon under study (e.g. a phylogenetic tree or network) part of the data input already. Not all authors have adopted the term ‘phylogeography’ for this approach but use others (e.g. historical biogeography) instead (Queiroz 2005, González- Wevar et al. 2010, McGaughran et al. 2011, Sanmartin 2012, Sanmartin et al. 2008, Arbogast & Kenagy 2008). Decades of internationally coordinated sampling have begun to ll in im- portant geographical as well as taxonomic gaps in our knowledge about the identity and distribution of the fauna in the Southern Ocean. The advent and widespread use of molecular tools on the other hand have allowed us to tap into unprecedented resolution of heritable traits within as well as among spe- cies. In concert, new tools and an increased availability of samples have led to a vastly increased knowledge about the species inventory and their distribu- tion since the synopsis of Hedgpeth (1969). One of the most popular applications of phylogeography that results from widespread generation of molecular barcodes are phylogeography networks that integrate intraspeci c molecular variants, their relative abundances and the locations where they have been found into a single diagram. Since net- works and geographical patterns inside of species are covered in detail in the two phylogeographic case studies that follow, the main focus of this chapter on phylogeography lies on how exactly the incorporation of spatial distribu- tions into the context of molecular phylogenies has advanced our understand- ing of the processes that shaped the evolutionary history of Southern Ocean organisms.