Protein phosphorylation affects most, if not all, cellular activities in eukaryotes and is essential for cell proliferation and development. An estimated 30% of cellular proteins are phosphorylated, representing the phosphoproteome, and phosphorylation can alter a protein's function, activity, localization and stability. Recent studies for large-scale identification of phosphosites using mass spectrometry are revealing the components of the phosphoproteome. The development of new tools, such as kinase assays using modified kinases or protein microarrays, enables rapid kinase substrate identification. The dynamics of specific phosphorylation events can now be monitored using mass spectrometry, single-cell analysis of flow cytometry, or fluorescent reporters. Together, these techniques are beginning to elucidate cellular processes and pathways regulated by phosphorylation, in addition to global regulatory networks.