The advent of single molecule imaging technologies in 1990s made the dynamics of single molecule enzyme catalysis be successfully probed in real time in vitro. Ever since then, single molecule enzymology entered the golden age of rapid developing. Individual features of single enzyme molecule hidden in the overall average were discovered, and many new catalytic mechanisms were proposed. Single molecule enzymology sheds light on the dynamic interactions between enzyme and substrate or product, deepening the understanding of biochemical reactions.