The present invention relates to photo-activated disinfection. In particular, the present invention relates to photo-activated disinfection in the oral cavity. We describe a dental apparatus comprising a light source producing light at a predetermined wavelength and power, and optical transmission means to train the light from the light source onto at least one external side of a tooth. Preferably, the optical transmission means comprises at least one optical guide. In a preferred embodiment, the optical transmission means comprises a pair of substantially parallel elongate light guides each light guide having a distal end provided with a reflective surface inclined at 45 DEG to the direction of light transmission along the light guides, the reflective surfaces of the light guides being opposed such that light emitted from each light guide is parallel but opposite in direction. Preferably, the light source is a light emitting diode or array of light emitting diodes and the has a wavelength of from 550 to 690 nm, more preferably from 600nm to 680nm, even more preferably from 625 to 660nm.