The biological effects of low intensity ultrasound (US) in vitro; the mechanisms involved; and the factors that can enhance or inhibit these effects are reviewed. The lowest possible US intensities required to induce cell killing or to produce free radicals were determined. Following sonication in the region of these intensities, the effects of US in combination with either hyperthermia, hypotonia, echo-contrast agents (ECA), CO2, incubation time, high cell density or various agents were examined. The results showed that hyperthermia, hypotonia and microbubbles are good enhancers of the bioeffects, while CO2, incubation time and high cell density are good inhibitors. Cellular membrane damage is pivotal in the events leading to cell death, with the cellular damage-and-repair mechanism as an important determinant of the fate of the damaged cells. The optimal level of apoptosis (with minimal lysis) and optimal gene transfection efficiency were attained using a pulsed low intensity US. In summary, the findings suggest that low intensity US is potentially useful in therapy, while on the other hand, they also call for further investigation of such clinical scenarios as high-grade fever, edema or use of ECA which may lead to the lowering of the threshold for bioeffects with diagnostic US.