Increasing use of metal and metal oxide nanoparticles Me(O)NPs in products means many will inevitably find their way into marine systems. Their likely fate here is sedimentation following hetero-aggregation with natural organic matter and/or free anions, putting benthic, sediment-dwelling and filter feeding organisms most at risk. In marine systems, Me(O)NPs can absorb to micro-organisms with potential for trophic transfer following consumption. Filter feeders, especially bivalves, accumulate Me(O)NPs through trapping them in mucus prior to ingestion. Benthic in-fauna may directly ingest sedimented Me(O)NPs. In fish, uptake is principally via the gut following drinking, whilst Me(O)NPs caught in gill mucus may affect respiratory processes and ion transport. Currently, environmentally-realistic Me(O)NP concentrations are unlikely to cause significant adverse acute health problems, however sub-lethal effects e.g. oxidative stresses have been noted in many organisms, often deriving from dissolution of Ag, Cu or Zn ions, and this could result in chronic health impacts. •Nanoparticle (NP) use increasing, and NPs ultimately discharged to marine systems.•Metal ion dissolution from NPs causes oxidative stress at relevant concentrations.•Bioaccumulation & trophic transfer of NPs likely at all levels of marine food webs.•Biofilms & filter feeders are major NP accumulators, but many Classes lack study.•Current release levels unlikely to cause chronic damage, but may be a future issue. Exposure to metal (oxide) nanoparticles causes sub-lethal effects in marine organisms, the extent of which is related principally to the organisms' feeding regime, habitat and lifestyle.