Proteome provides highly valuable information on the amount, modifications, and subcellular localization of polypeptides. Accordingly, geneticists, molecular biologists, and biochemists have logically applied these new tools to respond to different lines of biological questions (inventory of proteins, impact of a mutation, dynamics of protein regulation under a given exposure, …). However, even if the results obtained are very informative, this approach needs an excellent experimental design which ensures robustness and thus yields reproducibility. The present chapter gives appropriate methods for assessing the proteome of Pseudomonas aeruginosa by using a two-dimensional gel electrophoresis approach. Protocols for crude protein extraction, protein separation by using immobilized pH gradients, and protein identification by Liquid Chromatography coupled with tandem Mass Spectrometry (LC-MS/MS) are given.