This paper describes an interactive system-COLUMBUS-for the development of well-structured programs in assembly language, Cobol, and PL/I. One of the basic concepts employed is the use of Nassi/Shneiderman charts as the unique reference document. For specifying the control structures of a program, the programmer uses a set of control primitives which are common for all languages used. Data declarations and executable statements are taken from the individual language. This mixture is called the COLUMBUS source form. From this source form Nassi/Shneiderman diagrams and cross-reference matrices can be produced in a language-independent fashion; pure source for the individual languages is produced by a set of preprocessors in a language-dependent fashion. This paper illustrates the mechanisms involved by giving some small examples for the individual languages, and it also gives a brief history of the project and reports on quantitative and qualitative results obtained so far. The plans for future extensions are also explained.