The marine environment acts as a sink for many hazardous substances, including persistent organic pollutants (POPs) and trace metals. Many of these compounds are toxic, persistent, bioaccumulative and can be prone to long range transport. Their sources and pathways into the marine environment are diverse with most hazardous substances being produced by anthropogenic activities. Some are, or have been, produced intentionally (such as brominated flame retardants, BFRs) whilst others are by-products of industrial processes (such as polycyclic aromatic hydrocarbons (PAHs) and dioxins). Direct or indirect release to rivers, from industrial discharges and from sewage works discharges, are a major source of a range of hazardous substances. Historically the biggest sources of hazardous substances to the Scottish environment were from heavy industry (steelworks, mining and gasworks), however, increased regulation and closure of some of these industries has reduced contaminant inputs from these sources. Run-off from urban areas and atmospheric deposition continue to be diffuse sources of hazardous substances to the marine environment. Due to the persistent nature of many hazardous substances, high concentrations can still be found in the sediments of industrialised and urbanised estuaries as a result of historical inputs. A number of POPs and trace metals have been highlighted as a cause for concern and have been the subject of extensive study and international regulation. Some contaminants have been monitored for many years and are often called legacy contaminants. Legacy contaminants include trace metals, polycyclic aromatic hydrocarbons (PAHs) and chlorobiphenyls (PCBs) but are still of concern, despite reductions in inputs, as they persist in the marine environment for many years. Others have only recently been highlighted as cause for concern and added to monitoring programmes and are often referred to as emerging contaminants. Examples of emerging contaminants are short chain chlorinated parrafins (SCCPs) and perfluorinated compounds (PFCs). Attempts have been made to create emission inventories for a number of hazardous substances. Breivik et al.1 created a global emission inventory for PCB congeners and Prevedouros et al.2 estimated a European consumption and atmospheric emissions inventory for pentabromo diphenyl ethers. This review is an update of the review undertaken in 20043 by Marine Scotland to identify the hazardous substances monitoring requirements, what hazardous substance data is currently held for the Scottish marine environment and identify any gaps in information that may require further monitoring or method development.