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In this thesis we explore the effects of chemical potentials or charge densities inside a thermal plasma, \nwhich is governed by a strongly coupled gauge theory. Since perturbative methods in general fail in this regime, \nwe make use of the AdS/CFT correspondence which originates from string theory. AdS/CFT is a gauge/gravity \nduality (also called holography), which we utilize here to translate perturbative gravity calculations into \nresults in a gauge theory at strong coupling. As a model theory for Quantum-Chromo-Dynamics (QCD), \nwe investigate N=4 Super-Yang-Mills theory in four space-time dimensions. This theory is coupled to \nfundamental hypermultiplets of N=2 Super-Yang-Mills theory. In spite of being quite different from QCD \nthis model succeeds in describing many of the phenomena qualitatively, which are present in the strong interaction. \nThus, the effects discovered in this thesis may also be taken as predictions for heavy ion collisions at the\nRHIC collider in Brookhaven or the LHC in Geneva. In particular we successively study the introduction\nof baryon charge, isospin charge and finally both charges (or chemical potentials) simultaneously.\n\nWe examine the thermodynamics of the strongly coupled plasma. Phase diagrams are given for the \ncanonical and grandcanonical ensemble. Furthermore, we compute the most important thermodynamical\nquantities as functions of temperature and charge densities~(or chemical potentials): the free energy, grandcanonical\npotential, internal energy and entropy. Narrow resonances which we observe in the flavor current spectral functions\nfollow the (holographically found) vector meson mass formula at low temperature. Increasing the \ntemperature the meson masses first decrease in order to turn around at some temperature and \nthen increase as the high-temperature regime is entered. While the narrow resonances at low temperatures can be \ninterpreted as stable mesonic quasi-particles, the resonances in the high-temperature regime are\nvery broad. We discuss these two different temperature-regimes \nand the physical relevance of the discovered turning point that connects them. Moreover, we find that flavor currents\nwith isospin structure in a plasma at finite isospin density show a triplet splitting of the resonances in the \nspectral functions. Our analytical calculations confirm this triplet splitting also for the diffusion pole, which is \nholographically identified with the lowest lying quasinormal frequency. We discuss the non-vanishing\nquark condensate. Furthermore, the baryon diffusion coefficient depends non-trivially on both: baryon \nand isospin density. Guided by discontinuities in the condensate and densities, we discover a phase transition \nresembling the one found in the case of 2-flavor QCD. Finally, we extend our hydrodynamic considerations\nto the diffusion of charmonium at weak and strong coupling. As expected, the ratio of the \ndiffusion coefficient to the meson mass shift at strong coupling is significantly smaller than\nthe weak coupling result. This result is reminiscent of the result for the viscosity to entropy density ratio, \nwhich is significantly smaller at strong coupling compared to its value at weak coupling.