Introduction
Oblique parameters are designed to constraint models of new physics from the electroweak precision observables. It is assumed that the effects of new physics only appear through vacuum polarisation and therefore lead to modified oblique parameters. Most of the effects on electroweak precision observables can be parametrised by three gauge selfenergy parameters (S, T, U) introduced by Peskin and Takeuchi [Phys.Rev.D46, 381409(1992)]. T measures the difference between the new physics contributions of neutral and charged current processes at low energies (i.e., sensitive to isospin violation).
 S (S+U) describes new physics contributions to neutral (charged) current processes at different energy scales.
 U is only constrained by the W boson mass and its total width. In addition U is generally small in new physics models. Therefore, the STU parameter space can often be projected down to a twodimensional parameter space in which the experimental constraints are easy to visualise.
 Changed R_{b} calculation following the publication JHEP 1208 (2012) 050 and Erratumibid. 1305 (2013) 074, see also [arXiv:1205.0299].
Determination of the oblique parameters
The constraints on the STU parameters are derived from a fit to the electroweak precision data, more details can be found in the SM section. Besides the STU parameters the floating fit parameters are M_{Z}=91.1875±0.0021 GeV, Δα_{had}(M_{Z}^{2})= 0.02757±0.00010, and α_{s}(M_{Z}^{2})=0.1191±0.0028. The following fit results are determined from a fit for a reference Standard Model with m_{t,ref}=173 GeV and M_{H,ref}=126 GeV:
S = 0.03 ± 0.10  
T = 0.05 ± 0.12  
U = 0.03 ± 0.10 
The correlation coefficients between the STU parameters are given by
S  T  U  

S  1  0.891  0.540 
T  1  0.803  
U  1 
Contours of 68%, 95%, and 99% confidence level in the TSplane. The reference point at which all oblique parameters vanish is defined by M_{H}=126 GeV and m_{t}=173 GeV. The small black line shows the SM prediction with the current precision on m_{t} and M_{H}, whereas the grey region shows the SM prediction with no assumption on M_{H}. 

Contours of 68%, 95%, and 99% confidence level in the TSplane, where U was constrained to 0 in the fit. The reference point at which all oblique parameters vanish is defined by M_{H}=126 GeV and m_{t}=173 GeV. The small black line shows the SM prediction with the current precision on m_{t} and M_{H}, whereas the grey region shows the SM prediction with no assumption on M_{H}. 

Testing new physics models with oblique parameters
Oblique parameters can be used to confront BSM models with constraints from electroweak precision measurements. Models which have been investigated by the Gfitter group are Inert Higgs and littlest Higgs models
 Models with large, universal or warped extra dimensions
 A fourth fermion generation
 Technicolour
 The TwoHiggsDoubletModel