Ranks and definability in superstable theories

We study the notion of definable type, and use it to define theproduct of types and theheir of a type. Then, in the case of stable and superstable theories, we make a general study of the notion of rank. Finally, we use these techniques to give a new proof of a theorem of Lachlan on the number of isomorphism types of countable models of a superstable theory.