Biological effects of ultrasound occur by one of four major mechanisms: heat, cavitation, microstreaming, and radiation force. These effects have been described in the laboratory (in vitro) but not in the clinical situation (in vivo). The issue of safety has been addressed repeatedly by the American Institute of Ultrasound in Medicine (AIUM), and threshold limits have been set for diagnostic ultrasound. Ultrasound instruments fall below the AIUM threshold intensities. Long-term effects of diagnostic ultrasound are reviewed; no effects have been discovered. Further research is recommended. The issues of routine ultrasound on normal patients versus indicated ultrasound is explored and clarified. At this point, routine ultrasound cannot be recommended. However, indicated ultrasound, especially in a high-risk population, has proven benefits including a reduction of perinatal mortality. A comparison is made between electronic fetal monitoring and diagnostic ultrasound. At present, no biological effects have been demonstrated from Doppler ultrasound. Recommendations are made in order to assure optimal management. Judging from some of the articles and statements in both the popular and clinical press, there is a large amount of bias and emotion over the National Institutes of Health (NIH) consensus statement on “the use of diagnostic ultrasound imaging in pregnancy.” Little attention is seemingly paid to the actual text of the report and the scientific literature on which it is based. The NIH statement lists the acceptable indications for the use of ultrasound. However, it does not recommend the routine use of ultrasound in pregnancy, which has led some authors to raise questions about the use of ultrasound in general and has led to concern among patients and practitioners. This paper clarifies some of the issues relating to the safety and use of ultrasound so that a more enlightened discussion can occur. In addition, it reviews some of the laboratory (in vitro) data on ultrasound effects, compares some of the in vitro data with the clinical (in vivo) data, and discusses the issues of output or intensity thresholds, bioiogical effects, routine ultrasound, indicated ultrasound, other forms of ultrasound such as electronic fetal monitoring, and informed consent.