This article focuses on the use of software in safety-critical systems. Computers are used in safety- critical applications. Software decisions are often based on economic rather than safety considerations. It is sometimes assumed that software is totally reliable since software does not wear out. Instead, the errors occur in a much more random and unpredictable manner. Software is a digital rather than analog artifact. As such, techniques like interpolation and extrapolation, used by many hardware engineers in calculations, do not apply. Changing a single bit in a computer program can have a very unpredictable effect on its operation or may be completely benign in some circumstances. The complexity of most software is such that it is extremely difficult to reason about its behavior with great certainty. The highest quality of personnel should be employed in the development of safety-critical applications. The engineers involved should be capable of absorbing the required toolkit of knowledge and also accurately comprehending the required operation of computer-based systems. No special qualifications are currently required for personnel developing or maintaining software for safety-critical systems. This contrasts with the more established engineering professions where standards, regulations, certification, and accreditation often apply much more strictly. INSETS: Formal Methods and Formal Specification; Code of Practice for Engineers and Managers.