  We know that giant planets played a crucial role in the making of our Solar
System. The discovery of giant planets orbiting other stars is a formidable
opportunity to learn more about these objects, what is their composition, how
various processes influence their structure and evolution, and most importantly
how they form. Jupiter, Saturn, Uranus and Neptune can be studied in detail,
mostly from close spacecraft flybys. We can infer that they are all enriched in
heavy elements compared to the Sun, with the relative global enrichments
increasing with distance to the Sun. We can also infer that they possess dense
cores of varied masses. The intercomparison of presently caracterised
extrasolar giant planets show that they are also mainly made of hydrogen and
helium, but that they either have significantly different amounts of heavy
elements, or have had different orbital evolutions, or both. Hence, many
questions remain and are to be answered for significant progresses on the
origins of planets.
