Global marine and coastal ecosystems have been reshaped many times over the past few million years as the result of glacial cycles. Understanding the effects of these climate shifts on marine ecosystems sheds light on the impacts of future climatic change. Population genomic and demographic tools have rapidly improved in recent years - honed on an abundance of genomic sequence data for model organisms. Many of these tools can be applied in marine environments and are providing an unprecedented ability to investigate changes in population size through time as well as both present day and historical connectivity. This information is being used to understand adaptation, to inform fisheries and conservation efforts, and even to constrain climate change predictions. Recent research from tropical to the polar marine environments will be presented to illustrate the power of applying modern genomic tools to understand change and connections through space and time.