The provision of software structures to enable a computer to tolerate faults is frequently proposed as an effective and economic method of improving the reliability of a computer system. The paper is concerned with assessing the ability of software structures to cope with induced faults. It discusses the design of a software harness which allows the user to insert predetermined faults into software and to trace the subsequent execution of that software. The paper summarizes the use of the harness to determine the effectiveness of software-based fault-tolerant mechanisms.