About 30 years ago two-dimensional gel electrophoresis (2DE) was developed independently by Klose and O'Farrell representing the combination of two orthogonal separation techniques. In the first dimension the proteins are separated by isoelectric focusing (IEF) according to their isoelectric point. In the second dimension proteins are separated according to their electrophoretic mobility by conventional SDS-PAGE. For IEF two different techniques, immobilized pH gradient (IPG) and carrier-ampholyte-based IEF (CA-based IEF), respectively, are currently applied. With a resolution of up to 10,000 protein spots in one gel, 2DE offers a huge potential to give a comprehensive overview of the proteins present in the examined system. In combination with image analysis and mass spectrometry 2DE is still the method of choice to analyse complex protein samples.In this chapter we provide detailed protocols for both 2DE systems and give an overview about the latest developments including the two-dimensional difference gel electrophoresis (DIGE) system.