1) Scenarios on the history of star formation that use only observations in the UV and optical rest-frames result in luminosity functions that are strongly biased in the high luminosity end.
2) The most luminous nuclear and off-nuclear starbursts are enshrouded
in dust. In merging galaxies ISO revealed off-nuclear starburst knots
with sizes 
100 pc that
produce bolometric luminosities of up to
2 × 1011
L
(e.g. NGC 3690). A few of these starburst
knots can produce the overall bolometric luminosity of an
ultraluminous galaxy such as Arp 220.
3) The observation with ISO of the nearest AGN to Earth (Centaurus A) opens the general question on whether the hosts of giant radio galaxies are symbiotic galaxies composed of spirals at the centre of giant ellipticals.
4) Mergers of disks can produce metal-rich elliptical galaxy cores.
5) Collisions between giant disk galaxies trigger the formation of dwarf galaxies out tidal debris. A fraction of these re-cycled galaxies become detached systems with diverse morphologies: blue compact dwarfs, dwarf irregulars, and irregulars of Magellanic type.
Acknowledgements: Most of the work review here was carried out in collaboration with D.B. Sanders, P-A. Duc, V. Charmandaris and O. Laurent.