The research described here concerns the design and construction of an empirically-based debugging aid for first-time computer users, integrated into the Open University's SOLO programming environment. Its basis is an account of the processes involved as human experts debug faulty code, which account was later found to be supported by empirical tests on human experts. The account implies that an understanding of the intentions of the programmer is not essential to successful debugging of a certain class of programs. That class comprises programs written in a database-dependent language by users who are initially completely computer-naive and who during their course become competent to write simple programs which embody one or more basic AI techniques such as recursive inference. The debugging system, called AURAC, incorporates an explicit model of the debugging strategies used by human experts. Its understanding, therefore, is of programming in general and of the SOLO environment in particular. We present in the process a broad taxonomy of naive users' errors, showing that they can be divided into types, each type requiring a different debugging approach and indicating a different degree of expertise on the part of the perpetrator. SOLO is a conveniently delimited though nonetheless rich problem domain. Also described is a new version of SOLO itself (MacSOLO) which incorporates a large number of traps for the simple errors which plague novices, thus enabling AURAC to concentrate on the more interesting programming mistakes. AURAC is intended to operate after the event rather than whilst a program is actually being written, and is able via analysis of programming cliches and of data flows to isolate errors in the user's code. Where AURAC cannot analyse, or where its analysis yields nothing useful, it describes the corresponding section of code instead, so that the user receives a coherent output. MacSOLO and AURAC together form a unified system, based upon the principles of Simplicity, Consistency and Transparency. We show how these principles were applied during the design and construction phases.