In this article the relevance of phase-shift interferometry as an in situ probe for interfacial dynamics in thin cells is emphasized. The efficiency of this technique for dynamical studies of diffusion layers is illustrated by selected examples of electrodeposition and corrosion experiments in thin cells. After a brief survey of the application of interferometry to electrochemistry we turn to phase shift interferometry (PSI) and we show successively that this real time technique can be used for probing both transport processes and chemical mechanisms close to an active surface (mum scales).