This book provides documentation for a new version of the S system released in 1988. The new S enhances the features that have made S popular: interactive computing, flexible graphics, data management and a large collection of functions. The new S features make possible new applications and higher-level programming, including a single unified language, user defined functions as first-class objects, symbolic computations, more accurate numerical calculations and a new approach to graphics. S now provides direct interfaces to the powerful tool of the UNIX operating system and to algorithms implemented in Fortran and C. This book should be of interest to students taking courses in statistical computing.