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From the earliest years of ferrite development, spin-lattice interactions from select transition-metal (3dn) and rare-earth (4fn) ions have influenced key parameters of microwave device performance. Hysteresis loop squareness and coercivity, and magnetic loss that is related to the ferrimagnetic resonance linewidth will continue to depend on cautious inclusion of ions of the 3dn and 4fn series. As the promise of films and layers for planar devices are made more realizable by rapidly advancing deposition technologies, new challenges appear from the presence of stress imparted by mismatches at film/substrate interfaces. To offset unwanted effects of bias strains, increased use of magnetoelastic ions will become necessary. An additional feature made possible by deposition processing at temperatures lower than those of bulk ceramic sintering or melt-grown single crystals is the stabilization of cation valence states that do not normally occur in the thermodynamic equilibrium of conventional bulk ceramic processing. In the future that will undoubtedly involve structures of steadily decreasing dimensions, ferrite applications will have to account for surface compositional variations and interface stress effects. To this end, this article examines the physics that underlies the possible advantages and potential hazards to microwave technologies of the future, where magnetoelastic 3dn ions Mn3+, Fe4+, Fe2+, Co4+, Co2+, and Cu2+, as well as other ions such as Ho3+ or Dy3+ of the 4fn shell can influence the microwave performance.