This chapter has the form of a postponed prelude for special relativity. A prelude because it describes some of the experimental and theoretical work done before 1905. Postponed because these subjects might properly have appeared in a first chapter, although we chose not to do that. So here they are: Sections 10.2-10.6 on the experiments relevant to finding the ether and the theories that tried to understand the failure of these experiments while remaining true to Galileo and to Maxwell.