Failure is undesirable in engineering. Explaining the root causes of failure and predicting them through mathematical models based on physical principles provides a convincing and solid basis for decision making. Using physics to explain failure can help overcome the shortcomings of life data analysis, which is sometimes criticized for lacking a solid physical foundation. In general, engineering systems consist of mechanical, electrical, electronic, and software components. This chapter addresses fundamental physical principles related only to mechanical and electronic systems, while omitting software, as it is not governed by physical laws. The concepts discussed in this chapter may be less familiar to some engineers and may not directly explain specific failures, but they can enhance the understanding of the fundamental nature of failures.