A review of the principles of liquid penetration into porous media is presented, with emphasis on penetration of small drops (e.g. inkjet drops) into thin porous media (e.g. paper). The discussion consists of two parts: first, criteria of penetration are introduced, then the kinetics of penetration is analyzed. It is shown that the criteria of penetration depend not only on the material wettability (in terms of the contact angle), but also on the size of the liquid drop and the thickness of the porous medium. The kinetics of penetration of a drop into paper is shown to be different than theoretically expected. This is explained in terms of redistribution of the liquid between the paper pores. In addition, a method for the characterization of porous media by the kinetics of liquid penetration is presented.