A theoretical investigation of the microstructure of solar wind stream interaction regions is presented. We discuss the role of several electrostatic kinetic instabilities which may be important within the stream interface and the compression region. Inside of 1 AU the interface is likely to be stable against the electrostatic streaming instabilities considered. Between 1 and 2 AU we argue that the interface will excite the magnetized ion-ion instability. The compression region is also found to be unstable beyond 1 AU, where the modified two-stream instability, beam-cyclotron instability, and ion-acoustic instability will be important in determining the structure of the compressive pulses as they evolve into forward and reverse shocks. We conclude that the modified two-stream instability and beam-cyclotron instability predominately play a role in heating the electrons to the threshold for the ion-acoustic instability. Various electrostatic plasma waves, ranging in frequency from the lower-hybrid to harmonics of the electron cyclotron frequency, would be produced by these instabilities. Their signature should also be seen by high time resolution measurements of the temperature of the various plasma species.