 The reason for the failure of full abstraction of the continuous semantics of PCF is the existence of functions whose evaluation requires parallel computation. We describe now some proposals for characterizing sequentiality of functions by means of properties related to the structure of the domains on which they are defined. This has been an area of intensive research toward the solution of the full abstraction problem for PCF, and some of the insights that emerged from it lead very naturally to the game models discussed in  Section 3.  In addition, the following summary of attempts at a characterization of sequentiality is also a very interesting demonstration of the expressive power of the language of partial order in the semantic analysis of programming concepts.