Protein purification is usually a multistep process. The protein of interest is extracted from the matrix in which it occurs, captured by crude but fast and cheap methods, purified by removal of most impurities and then polished by removal of the last trace impurities. During all these steps, one should keep book about the protein recovery and the specific activity (purity) obtained. Apart from some precipitation methods, most purification involves chromatography to separate proteins by size, charge, hydrophobicity or binding to metal ions, hydroxyapatite, antibodies or ligands.