Thermal　barrier　coatings　(TBCs)　are　widely　used　in　the　hot-section　components　of　gas-turbine　engines　to　allow　operation　at　higher　temperatures　(＞　1　200　degrees　C),　which　has　created　some　new　issues.　One　issue　is　the　spallation　and　premature　failure　of　TBCs　caused　by　calcium-magnesium-alumino-silicate　(CMAS)　deposits,　which　arise　from　entry　of　siliceous　debris　such　as　fly　ash,　sand,　dust,　and　volcanic　ash　into　engines.　Since　1953,　over　130　jet　aircraft　have　encountered　volcanic　ash　clouds,　with　varying　degrees　of　damage　and　endangering　the　lives　of　many　passengers.　The　2010　eruption　of　Eyjafjallajokull　volcano　in　Iceland　led　to　the　most　severe　air-traffic　disruption　since　World　War　II.　The　operational　response　produced　economic　losses　approaching　1.7　billion.　When　these　debris　enter　the　hot-section　airfoil,　they　melt　and　are　accelerated　from　low　speed　(similar　to　15　m　/　s)　to　near　supersonic　speed　(similar　to　300　m　/　s),　impacting　and　adhering　to　the　TBC　surface.　Even　with　only　a　few　molten　silicate　ash　droplets　adhering　to　the　surface　of　hot-section　airfoils,　an　initial　deposit　layer　can　form　and　large　melt　pockets　(several　cubic　centimeters　in　volume)　can　accumulate.　Such　deposits　can　1)　block　cooling　holes　and　air　flow　paths,　and　2)　react　with　the　top　coating　of　hot-section　airfoils.　Furthermore,　adhering　droplets　infiltrate　the　interior　of　TBCs　under　capillary　forces.　Due　to　the　thermal　gradient　and　thermal　cycling,　the　infiltrated　CMAS　solidifies　and　fills　in　the　microcracks,　pores,　and　grain　boundaries,　resulting　in　loss　of　strain　tolerance　and　increased　coating　stiffness.　For　traditional　7-8　wt.%　yttria-stabilized　zirconia　(YSZ)　material,　chemical　reaction　with　CMAS　destroys　the　phase　and　structure　stability.　YSZ　grains　dissolve　and　Y-depleted　ZrO2　grains　precipitate　due　to　the　relatively　low　solubility　of　Zr4+　compared　with　Y3+　in　melted　CMAS.　Upon　cooling,　the　newly　formed　grains　transform　from　tetragonal　(t)　to　monoclinic　(m)　phases,　accompanied　by　a　3%-4%　volume　expansion.　As　turbine　inlet　temperatures　improve　and　industry　production　grows,　TBCs　are　suffering　from　severe　CMAS　corrosion.　This　issue　limits　further　application　and　development　of　TBCs;　enhancing　anti-corrosion　performance　of　TBCs　has　become　a　concern.　Herein,　we　compare　the　room-temperature　and　high-temperature　properties　of　different　CMAS　and　study　the　failure　mechanism　of　TBCs　exposed　to　CMAS.　We　also　determine　the　most　effective　CMAS　protection　method.　The　results　show　that　the　chemical　compositions,　especially　the　Ca:Si　ratio,　of　CMAS　such　as　volcanic　ash,　dust　and　sand　are　different,　further　affecting　their　high-temperature　viscosities　and　melting　behaviors.　With　infiltration　of　molten　CMAS　toward　the　coating　interior,　chemical　reaction　occurs　between　them,　resulting　in　instability　of　the　coating　microstructure　and　properties,　and　failure.　Significant　methods　including　inert-layer,　rare-earth　doping　and　novel　materials　have　been　proposed　to　improve　the　CMAS　corrosion　resistance　of　TBCs.　The　research　and　future　development　directions　of　CMAS　corrosion　and　protection　are　proposed,　providing　a　reference　for　design　of　novel　TBCs.