The construction of compiler front and back-ends is understood well enough for a great deal of the work to be automated. This paper describes the RTL System, which helps construct the rest of the compiler---the optimizer---by providing a flexible set of classes with a large number of predefined algorithms that the compiler writer can customize. The RTL System differs from systems to construct compiler front and back-ends because it does not specify the optimizations with a specialized language, but is instead an objectoriented framework. This paper describes the framework and how it can be used to build a code optimizer. 1 Introduction Compiler front and back-ends are understood well enough for them to be generated automatically. A parser generator can create the parser from a grammar describing the language, and a code generator can be built automatically from a description of the machine. However, the construction of an optimizer is much less straightforward. This paper describes th...