This work is concerned with the synthesis and evaluation of integrated communication systems constructed specifically to perform in the presence of channel disturbances of the form encountered in multipathpropagation. In the communication system considered, information is conveyed from source to user by encoding it into electrical symbols at the transmitter and decoding it at the receiver. Statistical theory, including information theory, is used throughout the paper. Gross aspects of multiple-mode propagation and ionospheric scattering are discussed, and previous efforts to improve systems affected by these disturbances are reviewed. System capacities are computed for the transmission of bandlimited white Gaussian signals through fixed multiple-mode disturbances, and the optimum spectral distribution of transmitter power is found. In general, presence of additional paths does not upset the message-handling capabilities of the channel if suitable wide-band signals are used. Quasi-stationary path fluctuations are considered, and a technique for measurement of the multipath characteristic by the receiver is suggested. Capacities arefound for slow path delay fluctuations. Single-mode scatter propagation is considered as a complex multiplicative Gaussian process. The probability computing receiver is found analytically, and a physical realization is developed for small signal-to-noise ratios. Lower bounds to system capacity are found for a binary transmission through scatter. Incidental results are system capacities for a related multiplicative channel, and the capacity for a binary transmission through white Gaussian noise. *This Report is identical with a thesis submitted to the Department of Electrical Engineering in partial fulfillment of the requirements for the degree of Doctor of Science at The Massachusetts Institute of Technology.