Membrane　distillation　(MD)　is　a　promising　desalination　technology　for　water　recovery　from　high-salinity　solutions　such　as　RO　brine.　Despite　its　great　potential,　MD　has　not　been　industrially　realized　because　this　technique　generates　a　lower　flux　than　reverse　osmosis　processes　and　exhibits　a　lower　thermal　efficiency　than　multi-stage　flash.　Enhancing　the　flux　and　thermal　efficiency　of　MD　is　a　major　research　focus　for　membrane　researchers.　In　this　study,　the　principles,　transfer　mechanism　and　thermal　efficiency　characterization　of　the　MD　process　were　introduced.　Various　factors　affecting　the　thermal　efficiency　were　analyzed.　Several　heat　recovery　techniques　in　the　MD　field　that　have　been　developed　and　widely　employed　were　reviewed.　(C)　2015　Elsevier　B.V.　All　rights　reserved.