[en] In this thesis we study several aspects of decays based on the quark level transitions b→sνanti ν and b→sμ⁺μ^- as well as transition form factors for radiative and rare semileptonic B meson decays. The quark level transition b→sνanti ν offers a transparent study of Z penguin and other electroweak penguin effects in New Physics (NP) scenarios in the absence of dipole operator contributions and Higgs penguin contributions. We present an analysis of B→K*νanti ν with improved form factors and of the decays B→Kνanti ν and B→X_sνanti ν in the Standard Model (SM) and in a number of NP scenarios like the general Minimal Supersymmetric Standard Model (MSSM), general scenarios with modified Z/Z^' penguins and in a singlet scalar extension of the SM. The results for the SM and NP scenarios can be transparently visualized in a (ε;η) plane. The rare decay B→K*(→Kπ)μ⁺μ^- is regarded as one of the crucial channels for B physics as it gives rise to a multitude of observables. We investigate systematically the often correlated effects in these observables in the context of the SM and various NP models, in particular the Littlest Higgs model with T-parity and various MSSM scenarios and identify those observables with small to moderate dependence on hadronic quantities and large impact of NP. Furthermore, we study transition form factors for radiative and rare semi-leptonic B-meson decays into light pseudoscalar or vector mesons, combining theoretical and phenomenological constraints from Lattice QCD, light-cone sum rules, and dispersive bounds. We pay particular attention to form factor parameterizations which are based on the so-called series expansion, and study the related systematic uncertainties on a quantitative level. In this analysis as well as in the analysis of the b→s transitions, we use consistently a convenient form factor definition on grounds of helicity. (orig.)