Each technology requires a certain number of previously completed technologies as a prerequisite. It is possible to research all of the technologies available in a particular Era. Technologies can be queued - in this case research on the next technology in the queue begins automatically as soon as the previous one is completed.
Starting the game in a later Era or progressing in the campaign will automatically grant all technologies from the previous Eras. Advancing to the next Era in the same scenario will not grant any technologies.