Before two-dimensional electrophoresis (2-DE), proteins of the sample must be denatured, reduced, disaggregated, and solubilized. Sample solubilization is usually carried out in a buffer containing chaotropes (typically 9.5 M urea, or 5–8 M urea and 2 M thiourea), 2–4% nonionic and/or zwitterionic detergent(s), reducing agent(s), carrier ampholytes and, depending on the type of sample, protease inhibitors. In this chapter, the major constituents of sample solubilization/lysis buffers will be briefly reviewed, some general sample preparation guidelines will be given, and the most common protein solubilization cocktails will be described.