Hydrogen dynamics is related to many important physical, chemical, and biological processes. Tautomerization in organic molecules is an intriguing model system for investigating hydrogen dynamics. We have addressed real-space study of single-molecule tautomerization using state-of-the-art low-temperature scanning probe microscopy, which is now an emerging topic in the field and provides a unique opportunity to directly visualize hydrogen dynamics . Porphycene, a structural isomer of porphine, exhibits particularly interesting tautomerization dynamics due to the strong intramolecular hydrogen bonds, resulting in the low tautomerization barrier and strong anharmoncity in the potential energy surface, whereby nuclear quantum mechanical effects of hydrogen play a crucial role . We have demonstrated direct observation and control of single-molecule tautomerization in porphycene adsorbed on metal surfaces, induced by various external stimuli, namely heat , electron , light , and force  as well as occurring by quantum tunneling . We further showed that the tautomerization can be precisely controlled by a local environment on the atomic level . Our approach provides novel microscopic insights into hydrogen dynamics in heterogeneous systems, which serves as a basis for ab initio quantum mechanical simulations and for potential applications for single-molecule switch.