Submicron　and　nanostructured　body-centered　cubic　(BCC)　metals　exhibit　unusual　mechanical　performance　compared　to　their　bulk　coarse-grained　counterparts,　including　high　yield　strength　and　outstanding　ductility.　These　properties　are　important　for　their　applications　in　micro-,　nano-and　even　atomic-scale　devices　as　well　as　for　their　usages　as　components　for　enhancing　the　performances　of　structural　materials.　One　aspect　of　the　unusual　mechanical　properties　of　small-sized　BCC　metals　is　closely　related　to　their　dimensional　confinement.　Decreasing　the　dimensions　of　single　crystalline　metals　or　the　grain　sizes　of　polycrystalline　metals　contributes　significantly　to　the　strengthening　of　the　small-sized　BCC　metals.　In　the　last　decade,　significant　progress　has　been　achieved　in　understanding　the　plasticity　and　deformation　behaviors　of　small-sized　BCC　metals.　This　paper　aims　to　provide　a　comprehensive　review　on　the　current　understanding　of　size　effects　on　the　plasticity　and　deformation　mechanisms　of　small-sized　BCC　metals.　The　techniques　used　for　in　situ　characterization　of　the　deformation　behavior　and　mechanical　properties　of　small-sized　samples　are　also　presented.