Photoluminescence　(PL)　imaging　has　become　a　fundamental　tool　in　disease　diagnosis,　therapeutic　evaluation,　and　surgical　navigation　applications.　However,　it　remains　a　big　challenge　to　engineer　nanoprobes　for　high-efficiency　in　vivo　imaging　and　clinical　translation.　Recent　years　have　witnessed　increasing　research　efforts　devoted　into　engineering　sub-10　nm　ultrasmall　nanoprobes　for　in　vivo　PL　imaging,　which　offer　the　advantages　of　efficient　body　clearance,　desired　clinical　translation　potential,　and　high　imaging　signal-to-noise　ratio.　In　this　review,　we　present　a　comprehensive　summary　and　contrastive　discussion　of　emerging　ultrasmall　luminescent　nanoprobes　towards　in　vivo　PL　bioimaging　of　diseases.　We　first　summarize　size-dependent　nano-bio　interactions　and　imaging　features,　illustrating　the　unique　attributes　and　advantages/disadvantages　of　ultrasmall　nanoprobes　differentiating　them　from　molecular　and　large-sized　probes.　We　also　discuss　general　design　methodologies　and　PL　properties　of　emerging　ultrasmall　luminescent　nanoprobes,　which　are　established　based　on　quantum　dots,　metal　nanoclusters,　lanthanide-doped　nanoparticles,　and　silicon　nanoparticles.　Then,　recent　advances　of　ultrasmall　luminescent　nanoprobes　are　highlighted　by　surveying　their　latest　in　vivo　PL　imaging　applications.　Finally,　we　discuss　existing　challenges　in　this　exciting　field　and　propose　some　strategies　to　improve　in　vivo　PL　bioimaging　and　further　propel　their　clinical　applications.