Theoretical estimates of phase shifter performance as a function of material parameters are presented by examining the results obtained from the small perturbation model used by Button and Lax and from the fully ierrite loaded, parallel plane waveguide model used by Suhl «And Walker. The extension of Suhi's theory on nonlinear effects at high power by Fletcher and Silence is applied to relate the material parameters and operating frequency to the magnetic field required to avoid high power subsidiary resonance effects. Experiments designed to demonstrate the relationship of phase shift and loss characteristics to material parameters and configuration for icigitudinally magnetized ferrites in rectangular waveguide are described. Polycrystalline ferrites with both cubic and hexagonal structures are examined. Of the hexagonal materials,both uniaxial and planar types with oriented ani-so.ropy fields are discussed. The experimental work leading to the growth of extremely large single crystals of lithium ferrite and their electrical and magnetic properties are reported. Experiments which reduced the linewidth of lithium ferrite samples by heat treatment and polishing are described. High power tests to 100 kw peak are reported for waveguide configurations containing (1) single crystal lithium ferrite; (2) polycrystalline cubic structure, nickel ferrite; and (3) polycrystalline hexagonal structure nickel-cobalt "W" ferrite with its magnetic anisotropy oriented parallel to the applied magnetic field. These high power tests demonstrate that the nonlinear high power effects can be avoided by operating at magnetic fields above ferrimagnetic resonance as prescribed by Fletcher and Silence.