  Saharon Shelah developed Morley’s ideas with great resourcefulness and energy. His main aim was to stretch the ‘Few is beautiful’ idea by showing that there are clear dividing lines between kinds of theory T. On one side of a dividing line are theories with some good structural property that forces the number of nonisomorphic models of a given cardinality to be small. On the other side, every theory has (for example) two models of the same cardinality that are not isomorphic but are extremely hard to tell apart. Shelah coined the name classification theory for this research. The text of Lascar listed below is an elegant introduction to this whole programme, from Łoś to Shelah.  Meanwhile Shelah himself has extended it far beyond first-order logic.  Even in the first-order case, Shelah had to invent new set-theoretic techniques (such as proper forcing) to carry out his constructions.