Dual Polarisation Interferometry (DPI) is an important enabling tool for cell biology, particularly the study of membranes and membrane proteins. In particular the behaviour and interactions of proteins and peptides with lipid bilayers can be studied. This application note describes the use of DPI for the real time, quantitative analysis of proteinlipid interactions. The molecular system under investigation incorporates phospholipids commonly found in cell membranes and known to be involved in the regulation of a range of membrane proteins, PtdIns(4,5)P2 (D-myo-phosphatidylinositol 4,5-biphosphate known as PIP2).