Recent　emphasis　on　green　communications　has　generated　great　interest　in　the　investigations　of　energy　harvesting　communications　and　networking.　Energy　harvesting　from　ambient　energy　sources　can　potentially　reduce　the　dependence　on　the　supply　of　grid　or　battery　energy,　providing　many　attractive　benefits　to　the　environment　and　deployment.　However,　unlike　the　conventional　stable　energy,　the　intermittent　and　random　nature　of　the　renewable　energy　makes　it　challenging　in　the　realization　of　energy　harvesting　transmission　schemes.　Extensive　research　studies　have　been　carried　out　in　recent　years　to　address　this　inherent　challenge　from　several　aspects:　energy　sources　and　models,　energy　harvesting　and　usage　protocols,　energy　scheduling　and　optimization,　implementation　of　energy　harvesting　in　cooperative,　cognitive　radio,　multiuser　and　cellular　networks,　etc.　However,　there　has　not　been　a　comprehensive　survey　to　lay　out　the　complete　picture　of　recent　advances　and　future　directions.　To　fill　such　a　gap,　in　this　paper,　we　present　an　overview　of　the　past　and　recent　developments　in　these　areas　and　highlight　a　number　of　possible　future　research　avenues.