Protein function is highly regulated in pathways that are responsible for complex biochemical mechanisms such as growth, metabolism, and signal transduction. One of the most important mechanisms is posttranslational modification (PTM) changing protein surfaces by phosphorylation, sulfation, acetylation, methylation, glycosylation, and sumoylation resulting in a more than 100-fold higher complexity (Geiss-Friedlander and Melchior, Nat Rev Mol Cell Biol 8, 947-956, 2007; Hunter, Mol Cell 28, 730-738, 2007). This chapter presents a very efficient way to detect potential phosphorylation sites in protein families using overlapping peptides covering the complete primary structures (peptide scans) immobilized on glass slides. Results of kinase activity fingerprinting of cell lysates using peptide microarrays displaying peptide scans through all human peptidyl-prolyl-cis/trans-isomerases are shown.