Liquid?liquid diffusion, an ancient yet fundamental scientific problem, has not been fully described by theory due to the complex interactions between liquid molecules. As a result, precise experimental measurements are crucial for advancing our understanding of this phenomenon. Since the introduction of Fick's laws in 1855, various methods have been developed to measure the liquid?liquid diffusion coefficient. Among these, optical methods are particularly valuable for monitoring the diffusion process, offering more accuracy than non-optical techniques. In this paper, we begin by discussing the Fickian diffusion model and the commonly used methods for measuring liquid?liquid diffusion. We then present two types of non-Fickian phenomena observed in experiments, highlighting the limitations of current optical methods in monitoring diffusion. Finally, we introduce a new optical micro-sensor scanning method proposed by our research group, which offers a promising approach for studying and uncovering new principles of liquid?liquid diffusion.