This chapter charts the rise of oceanography as a discipline, from the first European-led age of discovery in the fifteenth century, to the scientific expeditions typified by HMS Challenger’s four-year voyage of 1872. Progress continued into the twentieth century with the increase of laboratories, research vessels and funding programs, such as the International Decade of Ocean Exploration (IDOE). Regional sites in key locations – including Hawaii, Bermuda and Ocean Station Papa in the Gulf of Alaska – have been complemented by satellite imagery of ocean statistics and marine life, starting with the Coastal Zone Color Scanner (CZCS), launched on the Nimbus-7 satellite, in 1978. As our ability to measure and analyze has increased, we are gaining a greater appreciation of the highly complex, interconnected nature of our oceans and the damage we are causing them. Rising sea levels have a profound effect on marine ecosystems, dramatically altering the physiology of organisms (e.g., coral reefs) that are unable to migrate or evolve to adapt to the changes. Such organisms face extinction. Meanwhile, loss of sea ice is altering pathways of ocean circulation, in addition to habitat loss. The sinking waters of subpolar Greenland are significant in the formation of the warm Gulf Stream Current; further disruption to this could alter Europe’s climate irrevocably. Increases in atmospheric CO2 levels are also affecting ocean acidity and salinity. This especially impacts the thousands of species that have evolved with calcified structures, and has potential ramifications for the physiology of all marine inhabitants. The chapter ends on a reflection that, whilst a change in plastic production and consumption may be a relatively easy change to make, there are increasingly significant threats to marine ecosystems. Not least of these is deep sea mineral exploration, to which microorganisms, which have survived for billions of years, may adapt, but we most certainly will not if the current changes continue unabated.