Malignant　tumor　(cancer)　remains　as　one　of　the　deadliest　diseases　throughout　the　world,　despite　its　overall　mortality　drops.　Nanomaterials　(NMs)　have　been　widely　studied　as　diagnostic　and/or　therapeutic　agents　for　tumors.　A　feature　of　NMs,　compared　to　small　molecules,　is　that　NMs　can　be　concentrated　passively　in　tumors　through　enhanced　permeability　and　retention　(EPR)　effect.　In　the　meantime,　NMs　can　be　engineered　to　target　toward　tumor　specific　markers　in　an　active　manner,　e.g.,　receptor-mediated　targeting.　The　relative　contribution　of　the　EPR　effect　and　the　receptor-mediated　targeting　to　NM　accumulation　in　tumor　tissues　has　not　been　clearly　defined　yet.　Here,　we　tackle　this　fundamental　issue　by　reviewing　previous　studies.　First,　we　summarize　the　current　knowledge　on　these　two　tumor　targeting　strategies　of　NMs,　and　on　how　NMs　arrive　to　tumors　from　blood　circulation.　We　then　demonstrate　that　contribution　of　the　active　and　passive　effects　to　total　accumulation　of　NMs　in　tumors　varies　with　time.　Over　time,　the　receptor-mediated　targeting　contributes　more　than　the　EPR　effect　with　a　ratio　of　3　in　the　case　of　urokinase-type　plasminogen　activator　receptor　(uPAR)-mediated　targeting　and　human　serum　albumin　(HSA)-mediated　EPR　effect.　Therefore,　this　review　highlights　the　dynamics　of　active　and　passive　targeting　of　NMs　on　their　accumulation　at　tumor　sites,　and　is　valuable　for　future　design　of　NMs　in　cancer　diagnosis　and　treatment.　©　Ivyspring　International　Publisher.