We present an extension to Standard ML, called SMLSC, to support separate compilation. The system gives meaning to individual program fragments, called units. Units may depend on one another in a way specified by the programmer. A dependency may be mediated by an interface (the type of a unit); if so, the units can be compiled separately. Otherwise, they must be compiled in sequence. We also propose a methodology for programming in SMLSC that reflects code development practice and avoids syntactic repetition of interfaces. The language is given a formal semantics, and we argue that this semantics is implementable in a variety of compilers. Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.