Electromagnetic ion cyclotron (EMIC) waves are generated by a fundamental plasma instability and interact with multiple particle populations in the Earth's magnetosphere. This dissertation describes the application of spacecraft data and linear theory of electromagnetic waves to investigating the evolution of EMIC wave properties in the presence of multiple magnetospheric ion species. In particular, the role of the low-energy heavy ion species on the wave properties is explored. A case study describes spacecraft measurements of EMIC wave activity, the multiple ion species (hot protons, cold protons, and cold He⁺) present during the wave activity, and the methods for performing thorough characterization and analysis of the wave observations. I show that the observed wave characteristics are not typical of such waves as established from linear cold plasma theory. By using the full range of the observations and applying them to modeling of linear wave growth, I then show that wave properties evolve in the presence of sufficient free energy (low density hot protons), high density cold protons, and warm He⁺ (~10 eV). Parametric study of linear wave growth using the observed multiple ion properties as a reference point implies that EMIC waves evolve due to these warm plasma effects of the heavier ion species, and may also evolve due to non-local generation and propagation. This motivates a global multi-spacecraft magnetospheric study of the dominant cold/warm ions (H⁺, He⁺, and O⁺) to establish their typical properties (composition, densities, and temperatures) at different magnetospheric locations and to determine where such cold/warm ions can lead to similar evolution of warm plasma EMIC waves assuming the hot proton free energy is available. I then apply these results to successfully explain typical growth rates and properties of EMIC waves observed in each MLT sector. The results show how our findings on cold/warm ion properties can be used in future studies of EMIC wave generation and properties, including the effect of the waves on scattering of relativistic radiation belt electrons, cold ion heating, and hot ion precipitation to the ionosphere resulting in the proton aurora.