Due　to　intensive　marine　activities　and　the　use　of　low-quality　fuel　oils,　the　marine　transport　accounts　for　a　considerable　part　of　air　pollution　in　the　transportation　sector.　Although　ships　provide　the　convenient　transportation,　they　exhaust　a　large　number　of　hazardous　pollutants,　especially　nitrogen　oxides　(NOx)　and　sulfur　oxides　(SOx),　which　have　a　significant　side　effect　on　environment　and　human　health.　To　alleviate　the　impact　of　global　shipping　on　the　environment,　international　maritime　organization　established　the　more　stringent　emission　regulations　from　Tier　I　to　Tier　III　in　order　to　reduce　emissions　from　ships.　As　a　result,　various　emission　reduction　technologies　need　to　be　developed　in　order　to　meet　more　stringent　regulations　in　the　future　and　reduce　the　pollutant　emissions.　Under　this　background,　it　is　indispensable　to　examine　the　existing　emission　reduction　technologies　when　exploring　another　possible　method　to　reduce　the　pollutant　emissions.　Based　on　a　significant　number　of　related　literatures,　it　is　general　to　utilize　the　marine　alternative　fuels　to　reduce　the　pollutant　emissions.　Especially,　liquid　natural　gas　(LNG)　is　considered　to　be　one　of　the　most　promising　marine　alternative　fuels　due　to　its　economy　and　environmentally　friendly　features.　This　review　thus　aims　to　summarize　the　different　emission　reduction　technologies　of　marine　diesel　engines　through　three　reduction　paths　of　fuel　optimization,　pre-combustion　control　and　exhaust　after-treatment.　Furthermore,　the　utilization　of　LNG　in　the　marine　diesel　engines　are　evaluated　comprehensively　from　three　aspects　of　environmental　protection,　energy　structure　and　economic　benefits.　At　the　end,　some　suggestions　on　the　future　research　are　given　based　on　the　extensive　review　on　the　state-of-the-art　literature.　©　2020　Elsevier　B.V.