Due　to　the　damage　they　have　caused　to　the　ozone　layer,　first-generation　refrigerants　known　as　chlorofluorocarbons　have　been　phased　out,　and　second-generation　refrigerants　known　as　hydrochlorofluorocarbons　are　being　phased　out.　Third-generation　refrigerants　known　as　hydrofluorocarbons　are　also　being　phased　out　due　to　their　high　global　warming　potential.　As　successors　to　the　above　three　generations　of　refrigerants,　hydrofluoroolefins　and　hydrochlorofluoroolefins　were　designed　and　created　in　laboratories.　They　have　been　attracting　increasing　research　interest　because　they　are　safe　and　environmentally　friendly.　In　this　paper,　various　aspects　of　the　recent　research　on　hydrofluoroolefin　and　hydrochlorofluoroolefin　refrigerants,　including　applications　in　refrigeration　systems　and　heat　pump　systems,　applications　in　organic　Rankine　cycles,　flow　patterns　and　heat　transfer　characteristics,　thermophysical　properties,　persistent　degradation　product,　toxicity,　flammability　and　ignitability,　thermal　stability　and　decomposition　mechanisms,　are　reviewed　and　discussed.　The　most　commonly　studied　hydrofluoroolefins　are　still　HFO-1234yf　and　HFO-1234ze(E).　HFO-1336mzz(Z)　is　also　worthy　of　further　attention　because　it　may　be　a　suitable　replacement　for　HFC-123.　Attempts　can　be　made　to　establish　a　dedicated　equation　of　state　for　commonly　used　hydrofluoroolefins　to　better　predict　the　thermodynamic　properties.　Further　studies　on　the　persistent　degradation　product,　toxicity,　flammability　and　ignitability,　thermal　stability　and　decomposition　mechanism　are　needed.