The rationale for using resin infiltration in carious lesions is based on the success achieved in obliterating the diffusion pathways of demineralised enamel with a low-viscosity resin. After treating enamel surface with an acid, rinsing and drying, resin is infiltrated into the porous enamel subsurface, filling the voids and, thus, cooperating on the arrestment of carious lesion progression. This microinvasive technique is indicated for treating non-cavitated approximal carious lesions in both primary and permanent teeth. Also, resin infiltration may be used aesthetically to improve the appearance of carious white-spot lesions and other hypomineralised lesions in anterior teeth (as fluorosis, molar-incisor hypomineralisation and other opacities), since successfully infiltrated enamel presents a similar optical behaviour as sound enamel. This chapter discusses the principles of the resin infiltration technique on the basis of the available scientific evidence and presents clinical applications of this treatment option.