Currently one of the most challenging tasks in biological and medical research is to explore and understand the function of all proteins encoded by the genome of an organism. A systematic approach based on the genome sequences is feasible because the full genome of many organisms presently is available and many more are underway. For the production of expression atlases different strategies are used. Early attempts to acquire information about protein expression levels have focused on the analysis of mRNA levels within different tissues and cell types. Recently, novel strategies to focus directly on protein levels have been developed. To assess global protein expression in a systematic and high-throughput manner, methods based on design of specific affinity ligands to recognize the proteins have been presented. By subsequently using these affinity molecules for detection of the corresponding proteins in a wide range of platforms, important information can be gained. This article focuses on strategies to profile protein levels and in particular the human protein atlas initiative and the use of microarray technologies.