The S Package System Stephen Kaluzny spk@insightful.com Insightful Corp. The latest release of S-PLUS includes a package system not unlike the package system in R. A goal of the system is the creation of source packages that can be installed both in R and S-PLUS. To achieve this goal many new low level functions often used by R packages have been added to S-PLUS and some existing functions now have additional arguments for R compatibility. An R.h include file allows most R package source code to be compiled under S-PLUS. A number of packages from CRAN have been modified to run under the S-PLUS package system. I will discuss the conversion process and describe areas that require further work. I will also show our package compatibility checker that examines a package source tree, identifying and possibly correcting non-portable areas.