This article, part of the Computational and Systems Biology themed issue, reviews the state of knowledge about large-scale measurements of absolute protein and mRNA expression levels and their correlation. The relationship between protein and mRNA expression levels provides insights into the combined outcomes of translation and protein degradation, which are crucial for gene expression regulation. The review discusses the mechanisms of translation and protein degradation, including the roles of translation initiation, elongation, and termination, as well as lysosomal degradation and ubiquitin-mediated proteolysis. It highlights the importance of sequence characteristics in regulating these processes, such as codon usage, mRNA secondary structures, and protein stability. The article also reviews methods for studying protein translation and turnover, including microarrays, RIP-Chip, TRAP, and ribosome profiling. The focus is on the protein-per-mRNA ratio, which is a key measure of the combined effects of translation and degradation. The review compares protein and mRNA concentrations across different organisms, noting that correlations vary widely, with bacteria showing the strongest correlations (R² = 0.20 to 0.47), yeast (R² = 0.34 to 0.87), and multi-cellular organisms (R² = 0.09 to 0.46). The article concludes by discussing the implications of these findings for understanding gene expression regulation and the potential for future research in this area.This article, part of the Computational and Systems Biology themed issue, reviews the state of knowledge about large-scale measurements of absolute protein and mRNA expression levels and their correlation. The relationship between protein and mRNA expression levels provides insights into the combined outcomes of translation and protein degradation, which are crucial for gene expression regulation. The review discusses the mechanisms of translation and protein degradation, including the roles of translation initiation, elongation, and termination, as well as lysosomal degradation and ubiquitin-mediated proteolysis. It highlights the importance of sequence characteristics in regulating these processes, such as codon usage, mRNA secondary structures, and protein stability. The article also reviews methods for studying protein translation and turnover, including microarrays, RIP-Chip, TRAP, and ribosome profiling. The focus is on the protein-per-mRNA ratio, which is a key measure of the combined effects of translation and degradation. The review compares protein and mRNA concentrations across different organisms, noting that correlations vary widely, with bacteria showing the strongest correlations (R² = 0.20 to 0.47), yeast (R² = 0.34 to 0.87), and multi-cellular organisms (R² = 0.09 to 0.46). The article concludes by discussing the implications of these findings for understanding gene expression regulation and the potential for future research in this area.