The equations describing the flow of an electrically conducting fluid in the presence of electric and magnetic fields are written down with the aid of certain simplifications appropriate to aeronautical applications. In order to estimate the probable significance of magneto-aerodynamic effects, some data on conductivity of pure and "seeded" air are first examined. Dimensionless quantities representing the ratios of forces and of currents are then formed and their values studied for conditions of flight in the atmosphere. Some examples of magneto-hydrodynamic and magnetogasdynamic effects in simple flows are given. These include two cases of Poiseuille flow of conducting liquids with applied magnetic fields and the case of quasi-one-dimensional gas flow with applied electrical and magnetic fields. In the last case, attractive possibilities are found for controlled acceleration or deceleration of gas at subsonic and supersonic speeds, even in constant-area channels. The behavior of the flow is characteristically different in different regimes of Mach Number and flow speed relative to certain "significant speeds" that are dependent on the ratio of electrical to magnetic field strengths. These are studied, and a chart is constructed to relate the length to the speed ratio of a maximum-acceleration constant-area channel. I t is concluded that the advantages that may accrue from magneto-aerodynamic methods are sufficiently attractive to justify the considerable research and engineering development that will be required. Among the unsolved engineering problems are the reduction of surface resistance of electrodes in contact with a conducting gas, development of techniques for seeding, and provision of the required magnetic fields in flight.