Previous Doppler images of evolved R.S CVn stars and of single pre-main-sequence stars reveal interesting differences in the types of magnetic activity seen in these two classes of objects and our Sun; the presence and nature of polar spots may be one of the most striking differences found. The time seems ripe now to extend the Doppler-imaging technique to the very cool end of the main sequence. There, rapidly-rotating stars are thought to be fully convective and have no convective overshoot layer as in the Sun and similar stars. Since this is exactly the region where the solar/stellar dynamo is supposed to be located, one could expect a fundamentally different field topology and thus a qualitatively different surface temperature distribution. However, recent magnetic images of main-sequence stars suggest no basic differences to active giants. In this paper I will discuss the basic principles of Doppler imaging and the observational requirements, its application to the cool dwarf stars YY Gem and LQ Hya and future applications to brown dwarfs. In principle, Doppler imaging also contains the possibility to image the transits of extra-solar planets down to the size of a few terrestrial radii.