An important reason for the high popularity of diagnostic ultrasound is its lack of hazardous side effects. However, as described below, the probability of inducing undesirable thermal and mechanical effects in tissue increases with a rise in the output of diagnostic ultrasound machines.The use of increasing negative peak pressures of the applied ultrasound pulse enhances the risk of inducing cavitation inside the body. The potential damage of cavitation results from the dynamic, e. g. wild pulsation, and a collapse of bubbles in the respective sound field. The collapse, especially, releases a high amount of energy, related to secondary effects like shock waves, jet formation and the formation of free radicals. To estimate the likelihood of cavitation and ensuing mechanical biological effects the so-called mechanical index was defined. Nowadays, the mechanical index is displayed in real-time on modern diagnostic ultrasound machines. Echo-contrast agents play an increasing role in sonography. These stabilised gas particles increase the likelihood of cavitation formation and the occurrence of other biological side effects. The potential for mechanically induced side effects of diagnostic ultrasound was tested in many animal studies. As for the use of ultrasound for diagnostic purposes, adverse side effects, like haemorrhage or rupture of capillaries, were only observed in the presence of tissue-gas interfaces - as found in the lung or the intestine - or with high concentrations of contrast media.Even though no adverse effects of diagnostic ultrasound have been reported in humans, potentially damaging effect of advanced ultrasound techniques cannot be denied. Various ultrasound organizations, e.g. WFUMB, have therefore formulated recommendations for the safe use of diagnostic ultrasound.