A VERY high altitudes, of the order of 20 miles or more, the earth's atmosphere becomes so rarefied that it no longer behaves as a continuous fluid. Proper account must be taken of the basic molecular structure of the air in the prediction of heat transfer and aerodynamic characteristics. The general field of rarefied gas dynamics, i.e., the mechanics of a gas so rarefied that mean path effects become important, was discussed by Tsien (l). The terms and free molecule were introduced to characterize phenomena associated with moderate and extreme rarefaction respectively. A slip flow is a flow in which the mean path is a small but not negligible fraction of the boundary layer thickness and thus corresponds to values of Mach number M and Reynolds number Re in the range 0.01 10, i.e., to a ratio of mean path to body dimension of approximately 10 or larger. The slip flow range would apply approximately to flight of a one-foot diameter missile, to take a specific example, at altitudes of the order to 20 to 50 miles, while molecule flow would obtain at altitudes above 80 miles. Important applications to instrumentation occur at very much lower altitudes. The general regions of flow, together with the ranges of Mach and Reynolds covered by the various experimental programs described subsequently, are indicated in Fig 1. Two experimental research groups—one at the Ames Laboratory of the NACA under the direction of Stalder and Goodwin, and one at Berkeley under the initial direction of Folsom and Kane—have been carrying on investigations in this area for the past nine years with particular emphasis on applications to high altitude, supersonic flight. Most of the work at Ames has been centered around problems of molecule flow, while at Berkeley, most of the emphasis has been on the slip flow range. A fairly complete summary report, together with an extensive bibliography, of the progress of research in the field through the summer of 1952 is contained in (2). I t is the purpose of this report to describe the principal results obtained at Berkeley during the period 19521955 which have application to high altitude aerodynamics. These investigations were carried out in the low density supersonic wind tunnel located at Berkeley and described in (2). Only those investigations which have not yet been published, or which have been published only in the form of project reports, will be discussed here. Other investigations during this period are listed in the bibliography of the present report.