In this chapter we will summarize the main photothermal, photoacoustic and photomechanical effects of coupling a laser beam into a material from the absorption of the laser light to the deactivation of vibrationally and electronically excited states. Some methods to estimate the resulting temperature rise will be discussed and the resulting pressure increase in the heated area explained. The relaxation of both pressure and thermal transients will be explored and several methods described, such as pump-probe spectroscopy and imaging techniques, which can be used to investigate the dynamics of the relaxation pathways. We will explain how photothermal effects can manifest as optical effects. Finally, we will describe how we can harness photothermally induced optical changes to provide a new methodology in bioimaging involving indestructible 5-10 nm noble metal nanoparticles that can be observed using photothermal tracking microscopy for unprecedented periods of time in live cell imaging.