Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) is based on the combination of two orthogonal separation techniques. In the first dimension, proteins are separated by their isoelectric point, a technique known as isoelectric focusing (IEF). There are two important variants of IEF, which are carrier-ampholine (CA)-based IEF and immobilized pH gradient (IPG)-based IEF. In the second dimension, proteins are further separated by their electrophoretic mobility using SDS-PAGE. Finally, proteins can be visualized and quantified by different staining procedures, such as Coomassie, silver, or fluorescence staining. This chapter gives detailed protocols for 2D-PAGE, using both CA- and IPG-based separation in the first dimension.