In order to elicit therapeutic effects, many drugs including small molecule anticancer drugs, proteins, siRNA, and DNA have to be delivered and released into the specific cellular compartments typically the cytoplasm or nucleus of target cells. Intracellular environment-responsive nanosystems that exhibit good extracellular stability while rapidly releasing drugs inside cancer cells have been actively pursued for effective cancer therapy. Here, we highlight novel designs of smart nanosystems that release drugs in response to an intracellular biological signal of cancer cells such as acidic pH in endo/lysosomal compartments, enzymes in lysosomes, and redox potential in cytoplasm and the cell nucleus.