The high spatial resolution offered by the Planetary Camera (WFPC2) of HST and the infrared camera (ISOCAM) of ISO has been exploited to detect and characterize the nucleus of comets even when they are active. Contrary to ground-based observations, the seeing is not longer the limiting factor. The signal from the surrounding coma, an extented source, decreases as the spatial resolution increases while that of the nucleus is preserved as the Point Spread Function (PSF) of the telescope. The central pixels of the cometary images, solar scattered light for WFPC2, thermal emission for ISOCAM, can therefore contain a significant component from the nucleus superimposed on that of the coma. The key problem is to correctly separate these two components. I shall review the various technics which have been applied to date to the HST and ISOCAM images and compare their relative merits. I shall present the results obtained on the eights nuclei detected so far, photometry, color, size and rotational state.