NIR-H2O2 is a cell-permeable near-infrared (NIR) fluorescent turn-on sensor. NIR-H2O2 has both absorption and emission in the NIR region. NIR-H2O2 responds to H 2 O 2 with a large turn-on NIR fluorescence signal upon excitation in the NIR region. NIR-H2O2 is capable of imaging endogenously produced H 2 O 2 in living cells and mice.