Maude has a standard library of predefined modules that, by default, are entered into the system at the beginning of each session, so that any of these predefined modules can be imported by any other module defined by the user. Also, by default the predefined functional module BOOL is automatically imported as a submodule of any user-defined module, unless such importation is explicitly disabled. We discuss below some of the basic predefined modules in the standard library; some of them have a syntax similar to that of their counterparts in OBJ3's standard prelude [27]. The META-LEVEL module is discussed in Section 2.5, and the LOOP-MODE module in Section 2.8. The entire standard library of predefined modules is included as Appendix D.