In　wireless　sensor　networks　(WSNs),　attackers　could　inject　false　data　into　the　networks　by　compromising　the　sensor　nodes.　False　data　injected　by　the　compromised　nodes,　if　undetected,　could　not　only　cause　false　alarms　but　also　consume　the　limited　energy　of　the　sensor　nodes,　posing　serious　threats　to　the　lifetime　of　networks.　To　mitigate　this　type　of　attacks,　a　number　of　en-route　filtering　schemes　to　filter　false　data　inside　the　networks　have　been　developed　in　the　past.　However,　there　is　lack　of　a　systematical　strategy　to　evaluate　those　schemes　and　establishing　a　foundation　for　designing　en-route　filtering　techniques.　To　address　these　issues,　we　compare　the　pros　and　cons　of　the　existing　en-route　filtering　schemes.　To　fairly　compare　the　performance　of　those　schemes,　we　conduct　theoretical　analysis　and　derive　a　set　of　closed　formulae　for　them.　Our　extensive　simulations　validate　our　findings.　Our　research　summarizes　the　state-of-art　research　development　and　lay　out　future　directions　in　this　area.