In this chapter the various FLI data types have been examined. FLI types perform a translation on data passed between Lisp objects and C objects, and there are two main sorts of FLI types: immediate and aggregate. Immediate types have a simple representation in computer memory, and represent objects such as integers, floating point number and bytes. Aggregate types have a more complicated structure in memory, and consist of structures, arrays, strings, and unions. Parameterized and encapsulated types were also discussed. Finally, a number of FLI types that perform specific functions, such as the :void type and the :wrapper type, were examined.