BOS is a prototype-based, object-oriented toolkit aimed at better supporting evolutionary software development. BOS attempts to support a spectrum of activities in one environment---ranging from rapid prototyping to code hardening. Features enabling rapid prototyping include a prototype-based object model, an interpreted language, run-time argument constraints, position and keyword arguments, and a user interface toolkit. BOS also provides features for code hardening such as multi-methods, multiple inheritance, external code wrapping mechanisms, and interfaces to other packages such as database management systems. BOS thus enables the end-to-end programming of software in an integrated and unified environment. BOS has been used to develop several full-size applications which have been evaluated and delivered externally.