Recent advances in next-generation DNA sequencing and proteomics have revolutionized our understanding of protein expression regulation. These technologies have enabled large-scale surveys of mRNA and protein abundances, revealing that steady-state protein levels are influenced by a combination of transcriptional, translational, and post-translational processes. The data show that mRNA concentrations alone do not fully predict protein levels, indicating the importance of post-transcriptional and translational regulation. Single-cell studies and evolutionary conservation further highlight the complexity and importance of these regulatory mechanisms. The article discusses recent technological advancements, such as mass spectrometry and high-throughput cell imaging, which have facilitated these studies. It also explores the relationships between mRNA and protein concentrations, the roles of different regulatory processes, and the conservation of protein abundances across species. The authors propose a model where mRNA expression acts as a switch, and post-transcriptional and translational regulation fine-tune protein levels. Despite the progress, there are still many unanswered questions, particularly regarding the specificity of translation regulation, feedback loops, and new mechanisms of protein abundance regulation.Recent advances in next-generation DNA sequencing and proteomics have revolutionized our understanding of protein expression regulation. These technologies have enabled large-scale surveys of mRNA and protein abundances, revealing that steady-state protein levels are influenced by a combination of transcriptional, translational, and post-translational processes. The data show that mRNA concentrations alone do not fully predict protein levels, indicating the importance of post-transcriptional and translational regulation. Single-cell studies and evolutionary conservation further highlight the complexity and importance of these regulatory mechanisms. The article discusses recent technological advancements, such as mass spectrometry and high-throughput cell imaging, which have facilitated these studies. It also explores the relationships between mRNA and protein concentrations, the roles of different regulatory processes, and the conservation of protein abundances across species. The authors propose a model where mRNA expression acts as a switch, and post-transcriptional and translational regulation fine-tune protein levels. Despite the progress, there are still many unanswered questions, particularly regarding the specificity of translation regulation, feedback loops, and new mechanisms of protein abundance regulation.