The COmputer STored Ambulatory Record (COSTAR) was developed at the Laboratory of Computer Science of the Massachusetts General Hospital (MGH). Since release into the public domain, COSTAR has become the most widely used automated ambulatory medical record system (AAMRS). However, because of limited acceptance in the health care community, neither COSTAR nor any other system can claim to be the standard automated record. Because of COSTAR's modular design, documentation, and private vendor support, it has been installed in diverse practice settings and has been the core of many experimental and innovative record developments. COSTAR has not remained a static piece of software but is an evolving system. The author briefly describes the history of COSTAR's development and reviews innovations in ambulatory practices using COSTAR.