Ultrasonic irrigant activation is the most widely used supplementary irrigation method. It relies on the oscillation of an instrument at ultrasonic frequency while surrounded by irrigant, which results in heating of the irrigant, intense streaming and, under certain conditions, cavitation. A variety of ultrasonic files, tips, wires and needles have been used for this purpose. The efficacy of ultrasonic activation depends on the size of the instrument, the power setting and the direction of oscillation. Sodium hypochlorite, ethylenediaminetetraacetic acid (EDTA) and chlorhexidine are the most commonly activated irrigants and they can be delivered either simultaneously or between activation periods. This method appears very effective in the removal of pulp tissue remnants and hard tissue debris from the root canal. However, there is conflicting information regarding its antimicrobial effect and there seems to be no improvement in the treatment success rate, at least in single-rooted teeth. Instrument-to-wall contact appears inevitable under clinical conditions and may result in oscillation dampening and removal of small amounts of dentine. Ultrasonic instruments may fracture during use because of fatigue. Irrigant extrusion through the apical foramen is very limited except when the irrigant is continuously delivered and activated by an ultrasonic needle. Electromagnetic interference with pacemakers is unlikely to occur. Important knowledge gaps still exist, so ultrasonic activation is a topic of interest for future studies. Keywords: acoustic streaming, cavitation, irrigation, ultrasonic activation