Protein purification is an important tool for investigations on protein function, structure analysis, and biotechnological use. Therefore a number of different techniques have been developed for fast, reliable, and reproducible overexpression and purification of relevant proteins. Affinity systems have been employed frequently due to speed, yield, and reduction of chromatographic steps necessary in order to get a highly purified protein. Over the years, different tags and matrices have been introduced to the scientific community, each providing a combination of advantages and disadvantages in the light of the protein of interest.