The size and shape dependent tunable electromagnetic (EM) properties of magnetite – Fe3O4 nanoparticles makes them an attractive material for various future electronics and biomedical device applications such as tunable attenuators, miniaturized isolators and circulators, RF antennas, EM shielding, and biomedical implants etc. The strategic design of RF devices requires specific dielectric and magnetic properties according to the applications, which in turn depends on the size and shape of the particles. At nanoscale, iron oxide’s magnetic and dielectric properties are very different from its bulk properties and can be tuned and enhanced by utilizing different synthesis approaches. In this chapter, we summarize electromagnetic properties of magnetite (Fe3O4) nanomaterials such as, complex permeability, complex permittivity, magnetic and dielectric loss tangents, saturation magnetization, temperature dependence, and ferromagnetic resonance; and how these properties can be optimized by varying different synthesis parameters. Finally, Fe3O4 nanocomposites will be explored by using different synthesis approaches for implementation of RF and microwave applications and we will conclude the chapter with future recommendations.