Protein histidine phosphorylation plays an important role in several key mammalian cellular signaling pathways but the enzymes that mediate histidine phosphorylation and dephosphorylation, the biological regulation of histidine phosphorylation, and the functional impacts of protein histidine phosphorylation are poorly understood. The Barrios laboratory is working to develop fluorogenic assays for monitoring histidine phosphatase activity, identify histidine phosphatase inhibitors, and understand the biological regulation and impact of histidine phosphorylation. Examples of each will be presented, along with context about how histidine phosphorylation might impact metalloprotein structure and function.