Dual Polarisation Interferometry (DPI) is an analytical technique used to understand the real-time structure and behaviour of a wide range of thin films and nanosurfaces through quantitative measurement. DPI has been successful across a plethora of surface science applications, including polymers, surfactants, fine chemicals and biomolecules. As an interferometric technique, DPI has a wide dynamic range so can accommodate a broad spectrum of solvents, such as DMSO and ethanol, and buffers and buffer additives. This means that the DPI user can carry out experiments in the conditions of choice rather than those dictated by the limitations of other techniques. The unique, absolute measurements provided by DPI enables the researcher to question and understand thin films and nanostructures and their interactions to an extent not previously available in a laboratory-based technique. Measurements of geometric molecular thickness and density enable researchers to probe structural changes to extremely high resolution (thickness sensitivity to 0.01 nm and surface concentrations of 0.1 pgmm-2) in real time. This presentation introduces the biophysical and surface analysis technique and discusses particular research areas as; nano-scale surface assembly, polymer-surfactant interactions, quantification of protein interactions with liposome and bilayer structures, biocompatibility studies, nano-particle characterisation and bionanotechnology; areas in which DPI has found particular success.