The global fit of the Standard Model to electroweak precision data, routinely performed by the LEP electroweak working groups and others, has been revisited in view of (i) the development of the new generic fitting package, Gfitter, (ii) the insertion of constraints from direct Higgs searches at LEP and Tevatron, and (iii) a more thorough statistical interpretation of the results. This paper describes the Gfitter project, and presents state-of-the-art results for the global electroweak fit in the Standard Model, and for a model with an extended Higgs sector. Example results are an estimation of the mass of the Higgs boson (M_H = 116.4^{+18.3}_{-1.3} GeV) and a forth-order result for the strong coupling strength (alpha_S(M_Z^2) = 0.1193^{+0.0028}_{-0.0027}(exp)+/-0.0001(theo)). Using toy Monte Carlo techniques the p-value of the SM has been determined (p=0.22). As an example of a New Physics model constraints are derived for the Two Higgs Doublet Model of Type-II using observables from the B and K physics sectors.
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