Robert Gentleman Modeling with Objects ********************* The 1991 revision of the S language introduced several new concepts. In this paper we show how three of these: dataframes, formulas, and object oriented programming can be used to simplify the manipulation of complex data. We consider the case where the data contain a longitudinal component and show how using an appropriate data structure provides an abstraction that allows these data to be handled and modeled in a very natural way. The ideas extend to other situations where components of the data collected have complex structure.