Low temperatures are frequently the limiting factor in the distribution of those native or introduced insects which must undergo a period of hibernation during the winter. In temperate climates all insects with the exception of those taking advantage of the heat and shelter provided by man, or of the body heat of warm blooded animals, undergo a more or less rigorous period of exposure to unfavorable conditions. It is desirable in the case of an established infestation or an expected outbreak of an insect pest to know how well it can resist winter temperatures. By means of this advance knowledge preparations for control, which are often expensive, may be made more intelligently. Low temperatures are used to advantage by entomologists in the cold-sterilization of infested stored products, mills, and dwellings. In temperate regions it is often convenient during the winter to expose infested goods or buildings to the cold atmosphere. A knowledge of the conditions required to make effective such an exposure is necessary. The present study deals with some of the more fundamental problems of insect freezing and seeks particularly to clarify in so far as possible at the present time the physical concepts involved in the freezing of insects. The insects used in the experimental work are representative of several types. Some insects, such as the wood borers and bark beetles, are exposed to extremes of winter temperature; others, as the box-elder bug, are exposed to moderately low temperatures in hibernacula, but often are insufficiently protected; and, lastly, insects such as most stored product pests are normally never exposed to low temperatures. As great a variety of insects as available was used in this work.