I just picked up and played MOO2 for the first time ever from GOG. So far the game is amazing but I have a noob question. How do I build Titans? I researched Titan Construction but when it was complete, the design does not show up in my colony build list. Am I missing something? Are there other technologies that are required? I have no research order or strategy and I am afraid I missed something when I was picking new techs in other fields.

Does the colony have a star base? I'm fairly sure colonies without star bases can only build Destroyers, or smaller, and support craft.
Also, if you're playing with the tactical mode on you'll need to design a ship that uses that size frame.

You have to change an existing design. You can only have 5 different designs to build from at any one time, but you can change a design that is being used/built and still keep the old ones.

Ahh Thanks Winged! That was it. I didn't realize I could only have 5 designs.