We describe a small set of additions to Scheme to support objectoriented programming, including a form of multiple inheritance. The extensions proposed are in keeping with the spirit of the Scheme language and consequently differ from Lisp-based object systems such as Flavors and the Common Lisp Object System. Our extensions mesh neatly with the underlying Scheme system. We motivate our design with examples, and then describe implementation techniques that yield efficiency comparable to dynamic object-oriented language implementations considered to be high performance. The complete design has an almost-portable implementation, and the core of this design comprises the object system used in T, a dialect of Scheme. The applicative bias of our approach is unusual in object-oriented programming systems. This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intelligence research is ...