Surface application has gained more and more popularity as a means of increasing the quality of paper and board. In its broadest sense, surface application covers everything from sur face-sizing to extrusion coating, but this review will concentrate on surface sizing and pigment coating, i.e. processes taking place as an integrated part of the paper manufacture. Because the goals of the surface treatmant are different and depending on the quality of the paper desired, the review will cover our present fundamental understanding of the processes themselves. The presentation will start with a description of the water or liquid penetration in paper. Then the mechanics of various blade coating processes and the liquid transport it blade coaters will be explained. Last will come a presentation of the size press and new equipment for surface sizing.