Bonuses: Trade for ships and analyze them, its great for gaining expensive colonizing tech. Just might get some population in the bargain. Trade for population that breathes a different atmosphere. Put them on their planet type, remove your pop from it, and that red star will turn green giving many more build spaces.
It is sometimes difficult to find out what another empire has so that you can ask for it in a trade. Look at any enemy designs you have seen in combat to look for ship technologies. Use long range scanners on recon ships to check designs you haven't yet met in combat (these won't be added to the enemy ship design list until you meet it in combat). Use the intelligence projects for Ship Blueprints, Tech Levels and Planet Info to find other techs.
You can trade with any empire that you have contact with regardless of the treaty in effect -- even during war!
Offering an AI empire a good trade can cause him to accept the trade. This will happen BEFORE he would declare war on you. This can be done for several turns even if you are a Mega Evil Empire. You will still get the normal treaty trade (because he didn't declare war). It can more than make up for the trade losses of 1-2K resources. (Parasite)
How an AI refuses a trade can give hints on his tech. "We do not have that tech" means OK, no fighters, Attack! Whereas "We don't want to trade for that" means Get on that PD tech research! (Parasite)
| Manual (SEIV)