This dissertation covers a series of experiments that realized, investigated, and controlled exciton-trion-polaritons in transition-metal dichalcogenides (TMDs). First, theoretical and experimental research that led to the established understanding of trions are reviewed. Then, recent theoretical developments that led to a complete reimagining of excitons, trions, and their interactions with photons is discussed. With the theoretical background established, detailed measurements of optical conductivity of TMD MoSe$_2$ are combined with quantitative analysis to determine the nature of strong Coulomb interactions between excitons and bound trions in TMDs. Then, design and simulations of, and measurements on a hybrid photonic-crystal-TMD structure is discussed whereupon remarkable agreement between experiment and theory evidences the existence of coherent exciton-trion-polariton formation. Initial experimental forays into electronic control of the optical properties of this polariton system is then reviewed. In the final chapter of this dissertation, a non-exhaustive investigation of potential device applications based on the understanding of exciton-trion-polaritons is conducted.