The confirmation of mRNA expression studies by protein chips is of high recent interest due to the widespread application of expression arrays. In this review the advantages, technical limitations, application fields and the first results of the protein arrays is described. The bottlenecks of the increasing protein array applications are the fast decomposition of proteins, the problem with aspecific binding and the lack of amplification techniques. Today glass slide based printed, SELDI (MS) based, electrophoresis based and tissue microarray based technologies are available. The advantage of the glass slide based chips are the simplicity of their application, and relatively low cost. The SELDI based protein chip technique is applicable to minute amounts of starting material (<1 microg) but it is the most expensive one. The electrophoresis based techniques are still under intensive development. The tissue microarrays can be used for the parallel testing of the sensitivity and specificity of single antibodies on a broad range of histological specimens on a single slide. Protein chips were successfully used for serum tumor marker detection, cancer research, cell physiology studies and for the verification of mRNA expression studies. Protein chips are envisioned to be available for routine diagnostic applications if the ongoing technology development will be successful in increase in sensitivity, specificity, costs reduction and for the reduction of the necessary sample volume.