Phosphoinositides are minor constituents of cell membranes playing a critical role in the regulation of many cellular functions. Recent discoveries indicate that mutations in several phosphoinositide kinases and phosphatases generate imbalances in the levels of phosphoinositides, thereby leading to the development of human diseases. Although the roles of phosphoinositide 3-kinase products and PtdIns(4,5) P 2 were largely studied these last years, the potential role of phosphatidylinositol monophosphates as direct signalling molecules is just emerging. PtdIns5 P , the least characterized phosphoinositide, appears to be a new player in cell regulation. This review will summarize the current knowledge on the mechanisms of synthesis and degradation of PtdIns5 P as well as its potential roles.