There are many situations in which it may be desirable to use XSB as a rule-processing subcomponent of a larger system, which is written in another language. To do this, one wants to be able to call XSB from the host language, often C, providing queries for XSB to evaluate and retrieving back the answers. An interface for calling XSB from C (or other language) is provided for this purpose and is described in this chapter. Simple examples of the use of this interface are given in the XSB/examples/c_calling_XSB subdirectory, in files cmain.c, cmain2.c, ctest.P, and Makefile.