ISO images of starbursts and active galaxies

We present some highlights from the mid-infrared (5-16 mu m) images of merg ers of massive galaxies obtained with the Infrared Space Observatory (ISO). We have observed: 1) ultraluminous infrared nuclei, 2) luminous dust-enshr ouded extranuclear starbursts, and 3) active galaxy nuclei (AGNs). In this contribution we discuss the observations of Arp 299, a prototype for very l uminous infrared galaxies, the Antennae which is a prototype of mergers, an d Centaurus A which is the closest AGN to Earth. From these observations we conclude the following: 1) the most intense starbursts in colliding system s of galaxies and the most massive stars are dust-enshrouded in regions tha t appear inconspicuous at optical wavelengths, 2) the most intense nuclear infrared sources are a combination of AGN and starburst activity, 3) the ho sts of radio loud AGNs that trigger giant double-lobe structures may be sym biotic galaxies composed of barred spirals inside ellipticals.