Although the application of cold has been known as an effective means of relieving pain and for the treatment of trauma and inflammation since the time of the ancient Greeks it was not until about 1961 that controlled cryosurgery became an acceptable technique in medicine. Since then it has found increasing application in almost all branches of medical science. This review discusses the generally accepted qualitative and quantitative reasons for the cryodestruction of in vitro cells during freezing and thawing and its relation to in vivo tissue destruction. Because the lethal freeze-thaw cycle is critically dependent on the cooling and warming rates a fairly extensive discussion of the theoretical methods used to determine cell thermal history is included. A short description of the different types of cryosurgical equipment in general use today is included along with the methods which have been used to monitor the extent of the cryolesion. The various areas of clinical application of cryogenic surgery are described together with the authors' views for future developments in this field.