The capillary is an attractive format for integrated microanalyses, which start with the injection of separate reactants into the capillary and their mixing inside the capillary. Due to the nonturbulent nature of flow inside the capillary, mixing reactants in a generic way is a challenging task. Three approaches have been suggested as a solution: mixing by electrophoresis, mixing by longitudinal diffusion, and, most recently, mixing by transverse diffusion of laminar flow profiles (TDLFP). This is the first review on TDLFP, describing: (i) the physical basis of the method, (ii) its theory, (iii) analytical and numerical solutions for the calculation of concentration profiles of mixed reactants, (iv) up-to-date applications, and (v) problems to be solved and future directions.