To meet the ongoing demand for higher data rates and greater user mobility, modern mobile communications systems increasingly employ adaptive antenna arrays. By moving antenna elements closer together, to fit them inside a cellular phone for instance, mutual coupling effects impair their radiation capabilities. To describe these impairments more descriptively in contrast to current approaches, the present thesis extends the familiar notion of radiation efficiency from a single radiator to arbitrary antenna arrays by introducing an orthogonal set of radiating degrees of freedom. Detailed examples illustrate the effects of mutual coupling. Decoupling and matching networks are introduced to counteract mutual coupling. Thus, a design method applicable to a broad class of antenna arrays is described and verified by numerous examples, thereby ohmic losses and narrow bandwidths are identified as major weaknesses of decoupling and matching networks in general. For an investigation of the influence of mutual coupling on a mobile diversity receiver system, closed-form expressions for its diversity gain are derived and discussed. The analysis is complemented by a comprehensive receiver noise model. Practical diversity and noise measurements confirm the validity of the theoretical concepts developed. The present work aims to convey a more descriptive understanding of radiator coupling and to raise awareness of the fact that aspects of the entire system must be accounted for for an objective assessment of the potentials of mutually coupled antenna arrays.