Luminescent　biosensing　in　the　second　nearinfrared　(NIR-II)　region　is　featured　with　superior　spatial　resolution　and　high　penetration　depth　by　virtue　of　the　suppressed　scattering　of　long-wavelength　photons.　Hitherto,　the　reported　NIR-II　nanoprobes　are　mostly　based　on　carbon　nanotubes,　organic　fluorophores　or　semiconducting　quantum　dots.　As　an　alternative,　trivalent　lanthanide　ions　(Ln3+)　doped　nanoparticles　have　been　emerging　as　a　novel　class　of　promising　nanoprobes.　In　this　review,　we　highlight　the　recent　progress　in　the　design　of　highly　efficient　Ln3+-doped　NIR-II　nanoparticles　towards　their　emerging　bioapplications,　with　an　emphasis　on　autofluorescence-free　bioimaging,　sensitive　bioassay,　and　accurate　temperature　sensing.　Moreover,　some　efforts　and　challenges　towards　this　rapidly　expanding　field　are　envisioned.　©　2019,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.