  In September of 1959, at the conference on Infinitistic Methods in Warsaw,
Ernst Specker presented a joint paper with Robert MacDowell in which the
authors proved that every model of Peano Arithmetic has an elementary extension
such that all new elements are larger than all elements of the model. Until
now, the theorem has been a constant source of new results and generalizations,
and there are still open problems related to it. This paper is a survey of all
those developments.
