In this chapter I present methods to measure stellar magnetic fields, in particular the regression method used with low-resolution spectropolarimeters and the Least Square Deconvolution method used with high-resolution spectropolarimeters. I discuss the advantages and inconvenients of each method and the various instruments available as of today. I conclude on the best polarimeter to use depending on the science goal and type of studied objects, and provide a short summary of the two ongoing large programmes MaPP and MiMeS.