Principal factors affecting the performance of microwave systems in composite and transient plasma channels are reviewed. Only macroscopic aspects of the channel characterization are considered. These include such effects as absorption, refraction, dispersion and depolarization; also nonlinear and random effects. An effort is made to assess their importance to communication with and tracking of rocketborne systems. It is concluded that channels incorporating, for example, the re-entry plasma sheath or the rocket exhaust are inaccessible at the present time to a detailed characterization. Further research, especially of experimental nature and extending beyond the traditional study o absorption loss, is indicated