The human genome project (1-3) initiated a dramatic change of the research strategy applied in life sciences from the punctual investigation of defined projects to global approaches. The 'omics' projects aim at studying the full characteristics of a given biological sample in a single set of experiments. Despite the impact of genomics in molecular biology (4) and medicine (5), the information encoded by genomes does not represent more than a rough ground plan for an organism. This information might restrict but does not describe the dynamics of the life processes occurring in cells and organisms. Healthy as well as disease status are governed by complex processes, which start from the activation of a (limited) number of genes located in the genome either on the same chromosome or in different chromosome segments. Gene activation, translation, transcription and maturation of mRNAs, as well as posttranslational modifications give rise to a multitude of mature proteins and peptides (6) with structures, which are not directly deducible from genome sequences. Although proteins and peptides are products of gene expression, there are much more different forms of functional proteins and peptides than genes encoding them. As a consequence of the tightly regulated steady state between synthesis, posttranslational modifications and degradation every cell, tissue and organ of an organism contains a complex pool of proteins, protein fragments and peptides adequately modified to fulfill their biological functions. This result in the equilibrium of different functional forms of a given protein in varying concentrations embedded into extremely complex interaction networks involving a multitude of other different proteins or peptides (Fig. 1). Because of the dynamic nature of gene expression, protein synthesis and posttranslational modifications, the identification and quantification of proteins alone is not sufficient to understand functional interactions. In this context it is important to realize that changes as small as the addition of a single phosphate, cleavage of a leader peptide, amidation or Progress in the field of proteomics, the branch of biology that studies the full set of proteins derived from a given genome, is moving fast. Two-dimensional gel electrophoresis (2DG) separation of complex protein mixtures and the subsequent analysis of isolated protein spots by mass spectrometry allow fast and accurate identification of proteins. The comparison of spots from different samples separated on customized 2D gels allows the detection of punctual differences in their mobility and facilitates tracing back differences in protein expression, presence of isoforms, splice variants and posttranslational modifications by mass spectrometry. In spite of significant analytical challenges owing to the high complexity of the proteome and the challenge deriving from the necessity to process huge amounts of raw data generated by mass spectrometric profiling, proteomics has evolved to an indispensable tool in life sciences. A restricte...