Competitive　advantage　today　is　no　longer　solely　measured　by　an　abundance　of　natural　resources　or　material　production.　It　is　generally　acknowledged　that　intangibles　such　as　knowledge　now　lead　to　firms　being　more　flexible　and　thus　more　competitive.　To　understand　the　development　of　competitive　advantage,　we　must　understand　knowledge　especially　tacit　knowledge　and　how　it　is　shared.　Therefore,　based　on　review　of　critical　literatures,　the　author　first　summarizes　current　views　of　the　definitions　of　knowledge　to　six　perspectives　and　classifies　views　of　tacit　knowledge　to　four　types.　This　paper　aimed　to　set　a　theoretical　foundation　for　studies　on　the　sharing　of　tacit　knowledge　in　organizations.　The　author　found　that　there　are　two　different　views　on　sharing　tacit　knowledge　due　to　the　conceptual　differences　of　it.　Then　the　author　investigate　the　difficulty　of　sharing　tacit　knowledge　and　found　that　the　difficulties　in　the　externalizing　and　sharing　of　tacit　knowledge　are　linked　to　perception,　language,　time,　value　and　distance.　Among　which,　perception　and　language　are　the　main　difficulties　in　sharing　tacit　knowledge.　©2009　IEEE.