Proteomic analysis is a powerful tool to follow physiological modifications and phenotypes of mesenchymal stem cells (MSC). This approach generates informative data on expression and post-translational modifications of proteins which are of interest to assess the true potential of MSC in regenerative medicine. No matter the technologies used, proteomic analysis is always a challenge as the proteome is extremely diverse (in terms of constituents and concentrations), is changing with time, and is highly sensitive to pre-analytical conditions. In the framework of a European project (GENOSTEM http://www.genostem.org/), we have set up a multisite two dimensional gel electrophoresis (2DE) proteomic comparison of MSC. The goal is to compare cells from different origins, to follow their differentiation and to ultimately define a specific MSC proteomic signature. One important initial task is the optimization of 2DE protocols such that they are robust enough to be used in a multisite project. In this chapter, we detail these protocols which can be used not only for MSC but also for other cells in culture.