Measurements with Los Alamos Scientific Laboratory instrumentation aboard Imp 6, 7, and 8 reveal that approximately one third of all high-speed solar wind streams observed at 1 AU contain a sharp boundary (of thickness less than ~4¿104 km) near their leading edge, called a stream interface, which separates plasma of distinctly different properties and origins. Identified as discontinuities across which the density drops abruptly, the proton temperature increases abruptly, and the speed rises, stream interfaces are remarkably similar in character from one stream to the next. A superposed epoch analysis of plasma data has been performed for 23 discontinuous stream interfaces observed during the interval March 1971 through August 1974. Among the results of this analysis are the following: (1) a stream interface separates what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas; (2) the interface is the site of a discontinuous shear in the solar wind flow in a frame of reference corotating with the sun; (3) stream interfaces occur at speeds less than 450 km s-1 and close to or at the maximum of the pressure ridge at the leading edges of high-speed streams; (4) a discontinuous rise by ~40% in electron temperature occurs at the interface; and (5) discontinuous changes (usually rises) in alpha particle abundance and flow speed relative to the protons occur at the interface. Stream interfaces do not generally recur on successive solar rotations, even though the streams in which they are embedded often do. At distances beyond several astronomical units, stream interfaces should be bounded by forward-reverse shock pairs; three of four reverse shocks observed at 1 AU during 1971--1974 were preceded within ~1 day by stream interfaces. Although stream interfaces often occur in close proximity to reversals in direction of the interplanetary magnetic field, the field reversals generally precede the interfaces by 1(1/2) hours to 1(1/2) days. Approximately 40% of stream interfaces at 1 AU produce si- in the geomagnetic field. It has been suggested previously that stream interfaces are the result of the nonlinear evolution of high-speed streams produced by a smoothly varying temperature elevation in the solar envelope. Our measurements appear to contradict this thesis: many streams do not appear to be smoothly varying close to the sun, and a temperature elevation in the solar envelope is not required to explain the abrupt jumps in plasma properties which occur at stream interfaces. Our observations suggest that many streams close to the sun are bounded on all sides by large radial velocity shears separating rapidly expanding plasma from more slowly expanding plasma. The shear at the leading edge of such streams becomes the stream interface observed at 1 AU; however, momentum transfer across the interface reduces the magnitude of the speed jump across the shear with increasing distance from the sun. The large abrupt increases in electron and proton temperature observed at interfaces are primarily a consequence of the fact that interfaces separate what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas.