Pollution enters the marine environment  through dumping, through the discharges from the operation of ships, through land-based sources and through the atmosphere.  The GESAMP has determined that the main culprit of marine pollution is land-based sources.  Industries, cities with limited sewage treatment capacity, and agricultural fields that are maintained by fertilizers and various chemicals often discharge pollutants into the rivers that end up in the seas.

A GESAMP report issued in 2001 has placed emphasis on the increasing global problem of eutrophication that is the increased biological production in costal and near shore waters because of the input of nutrients from sewage and agricultural fertilizers.  The GESAMP has  further determined that sewage is a problem of high priority in all regional seas.  After sewage environmental issues that need to be addressed with urgency include sediment mobilization, Persistent Organic Pollutants (POPs), heavy metals and physical alteration.  It has been determined further that low-level nuclear waste disposed of at sea is not as grave an issue as high-level radioactive waste that comes for the decommissioning of nuclear weapons, nuclear military installations and obsolete nuclear vessels.  There have been a number of accidents involving nuclear-powered and nuclear armed vessels.

Pollution from vessels, for instance ships that carry hazardous chemicals and tankers, is a small percentage of marine pollution (12 percent of the total), however, ships are often viewed as the main contributors to marine pollution because of the large  publicity that oil spills generate.  According to GESAMP certain marine environments are more sensitive to pollution than others and, thus, require special attention such as coral reefs, sea-grass beds, coastal wetlands, mangrove forests and shallow coastal waters.

A number of instruments have been adopted to deal with pollution of the seas including:
The United Nations Convention on the Law of the Sea (1982), a global umbrella convention that address a number of matters with regard to the regulation of the marine environment including pollution from all sources.

Two international conventions that deal with specific issues, the London Dumping Convention that deals with dumping of wastes at sea  and the MARPOL Convention that deals with pollution generated by ships.
A number of the conventions address pollution in regional seas
–the Convention for the Protection of Marine Environment of the North East Atlantic-OSPAR; the Helsinki Convention for the Protection of the Baltic Sea-HELCOM. The UNEP Regional Seas Program has spearheaded many efforts for the protection of regional seas: Antarctic, Arctic, Arabian Gulf, Black Sea, Baltic Sea, Caspian Sea, Northeast Atlantic, Northeast Pacific, Caribbean, West and Central Africa, East Africa, Red Sea and Gulf of Aden, Pacific, Northwest Pacific, Southeast Pacific, East Asian Seas, South Asian Seas, Mediterranean.

In 1975, the Mediterranean States and the European Community approved the Mediterranean Action Plan (MAP), the first action plan to address marine environmental degradation.  In this context, the Mediterranean Programme for International Environmental Law and Negotiation (MEPIELAN CENTRE) works closely with the UNEP to strengthen the institutional infrastructure in the region.