Fluorescence　imaging　technology　has　gradually　become　a　new　and　promising　tool　for　in　vivo　visualization　detection.　Because　it　can　provide　real-time　sub-cellular　resolution　imaging　results,　it　can　be　widely　used　in　the　field　of　biological　detection　and　medical　detection　and　treatment.　However,　due　to　the　limited　imaging　depth　(1-2　mm)　and　self-fluorescence　background　of　tissue　emitted　in　the　visible　region　(400-700　nm),　it　fails　to　reveal　biological　complexity　in　deep　tissues.　The　traditional　near　infrared　wavelength　(NIR-I,　650-950　nm)　is　considered　as　the　first　biological　window,　because　it　reduces　the　NIR　absorption　and　scattering　from　blood　and　water　in　organisms.　NIR　fluorescence　bioimaging＇s　penetration　is　larger　than　that　of　visible　light.　In　fact,　NIR-I　fluorescence　bioimaging　is　still　interfered　by　tissue　autofluorescence　(background　noise),　and　the　existence　of　photon　scattering,　which　limits　the　depth　of　tissue　penetration.　Recent　experimental　and　simulation　results　show　that　the　signal-to-noise　ratio　(SNR)　of　bioimaging　can　be　significantly　improved　at　the　second　region　near　infrared　(NIR-II,　1,000-1,700　nm),　also　known　as　the　second　biological　window.　NIR-II　bioimaging　is　able　to　explore　deep-tissues　information　in　the　range　of　centimeter,　and　to　obtain　micron-level　resolution　at　the　millimeter　depth,　which　surpass　the　performance　of　NIR-I　fluorescence　imaging.　The　key　of　fluorescence　bioimaging　is　to　achieve　highly　selective　imaging　thanks　to　the　functional/targeting　contrast　agent　(probe).　However,　the　progress　of　NIR-II　probes　is　very　limited.　To　date,　there　are　a　few　reports　about　NIR-II　fluorescence　probes,　such　as　carbon　nanotubes,　Ag2S　quantum　dots,　and　organic　small　molecular　dyes.　In　this　paper,　we　surveyed　the　development　of　NIR-II　imaging　contrast　agents　and　their　application　in　cancer　imaging,　medical　detection,　vascular　bioimaging,　and　cancer　diagnosis.　In　addition,　the　hotspots　and　challenges　of　NIR-II　bioimaging　are　discussed.　It　is　expected　that　our　findings　will　lay　a　foundation　for　further　theoretical　research　and　practical　application　of　NIR-II　bioimaging,　as　well　as　the　inspiration　of　new　ideas　in　this　field.