Solar　energy　is　assumed　to　provide　an　everlasting　solution　to　the　modern　energy　crisis　since　it　is　the　most　abundant　source　of　renewable　energy　and　solar　energy　harvesting　systems　do　not　generate　any　harm　to　the　environmental　stability　as　well.　Current　research　trends　on　solar　energy　are　mainly　focused　on　solar　thermal　energy　harvesting　systems　because　of　their　huge　potential　for　domestic　as　well　as　industrial　applications.　The　efficacy　of　a　solar　thermal　energy　harvesting　system　is　dependent　on　its　design　aspects　(i.e.,　geometry　and　material)　and　the　fluid　used　for　photothermal/photovoltaic-thermal　conversion　and　subsequent　thermal　energy　transportation.　Considering　the　crucial　role　of　the　working　fluid,　ample　research　is　underway　on　the　development　of　new　fluids　having　superior　optical　and　thermophysical　characteristics.　One　of　the　recently　developed　thermal　fluids　called　hybrid　nanofluids　possesses　high　thermal　potential　which　makes　them　a　suitable　candidate　for　application　in　solar　energy　systems.　This　review　article　contains　a　brief　discussion　on　important　characteristics　of　hybrid　nanofluids　and　a　summary　of　the　major　findings　of　recent　research　studies　that　have　tested　hybrid　nanofluids　in/for　solar　thermal　energy　harvesting　systems.　Considering　the　findings　of　reviewed　articles,　the　most　important　aspects　of　hybrid　nanofluids　influencing　the　performance　of　hybrid　nanofluid　based　solar　thermal　energy　harvesting　systems　are;　optical　characteristics,　thermal　conductivity,　viscosity,　pressure　drop,　nanoparticle　size,　nanoparticle　type,　nanoparticle　shape,　nanoparticle　concentration,　base　fluid,　nanoparticle　suspension　uniformity,　working　temperature　and　flowrate,　compatibility　of　individual　nanoparticles,　preparation　method,　period　and　power　of　magnetic　stirring　and　ultrasonication,　surfactant,　nanoparticle　agglomeration/clustering,　and　sedimentation.　Extensive　review　of　literature　reveals　that　the　inclusion　of　hybrid　nanofluids　can　improve　the　performance　of　solar　energy　harvesting　systems　by　two　times　at　optimal　operational　conditions.　The　reported　results　showed　that　metal　oxide　and　nano　carbon-based　hybrid　nanofluids　exhibit　higher　enhancement　in　energy　efficiencies　as　compared　to　other　nanofluids　under　the　same　conditions.　Despite　of　obvious　advantages　of　using　nanofluids　in　spite　of　conventional　fluids,　there　are　some　limitations　as　well　such　as　clogging　in　the　pipes,　and　thermal　hammering　in　pipes　due　to　extreme　cycles　of　temperature　variation.　Moreover,　the　use　nanoparticles　can　cause　some　environmental　problems　such　as　suspension　of　nanosized　metallic　particles　in　the　air　that　would　eventually　cause　air　quality　degradation　and　toxicity　in　the　air.　The　article　eventually　presents　some　important　recommendations　for　future　research.　©　2021　Elsevier　Ltd