This digest examines how the concepts of least restrictive environment, mainstreaming, and inclusion have been developed by Congress and the courts. An introductory discussion of the Individuals with Disabilities Education Act (IDEA) notes that Congress did not define the concept of least restrictive environment, but left it to the courts to shape a definition. A court definition of mainstreaming is provided. The paper then presents brief abstracts of Circuit Court cases that have helped to shape these concepts. The court cases are: Roncker v. Walter, (1983); Daniel R.R. v. State Board of Education (1989); Greer v. Rome (1991); Oberti v. Clementon (1993); Sacramento v. Rachel H. (1994); Light v. Parkway (1994); Clyde K. v. Puyallup (1997); Hartmann v. Loudoun (1997); Hudson v. Bloomfield Hills (1997); and Doe v. Arlington County (1999). Each case is referenced to the Federal Reporter or the Federal Supplement. (DB)