Photothermal excitation is an alternative cantilever drive mechanism for tapping mode atomic force microscopy. It uses a power-modulated laser focused on the cantilever to directly drive its oscillation, producing cantilever tunes that match the thermal response almost perfectly. This enables quantitatively accurate AFM imaging and viscoelastic nanomechanical mapping in all environments. In addition, the clean cantilever tunes are stable with time, even in liquids, and therefore result in exceptional imaging stability. Lastly, the ease of use of photothermal excitation and compatibility with small high-frequency cantilevers allows high-speed imaging in both air and liquid environments. We explain the advantages of photothermal excitation in these contexts, and contrast its benefits to problems associated with the well-established piezoacoustic drive mechanism.