Tautomerization in a single organic molecule is a fascinating model to study hydrogen dynamics and has recently obtained increasing attention in the field of nanoscale science because the process resembles a single-molecule switch. We have studied tautomerization of porphycene (a structural isomer of free-base porphyrin) by using low-temperature scanning probe microscopy (LT-SPM). Porphycene exhibits particularly interesting tautomerization behavior due to its strong intramolecular hydrogen bonds, and the investigation with LT-SPM opens a unique opportunity for studies of hydrogen-bond dynamics at the single-molecule level. I will discuss direct observation and control of tautomerization in the porphycene molecule on metal surfaces induced by different external stimuli-heat, tunneling electron, light, and force.