Activatable　photosensitizers　(aPSs)　are　a　new,　switchable　smart　photosensitizer　(PS)　that　can　enhance　photodynamic　therapy　(PDT)　specificity　and　efficiency.　aPSs　are　inert　in　healthy　tissues,　but　can　be　switched　on　by　neoplastic-associated　endogenous　stimuli　such　as　pH,　enzymes,　and　glutathione　(GSH)　and　reduce　damage　to　normal　cells.　This　review　summarizes　aPS-based　design　strategies　based　on　energy　or　electron　transfer　and　self-quenching.　We　supplement　the　advanced　design　strategies　with　newly-developed　molecular　aPSs　and　focus　on　introducing　nano-engineering　supramolecular　aPS　construction.　©　the　Partner　Organisations.