This chapter discusses the problems of detecting pipeline leaks, spills, and ruptures at a high level. The problem identifying drivers who are trying to avoid paying fees on a toll road is used as an analogy for pipeline leak detection to illustrate many of the concepts and issues that are discussed in more detail later. The concepts explored include approaches that detect losses from outside of the system and other approaches that utilize variables that can be measured from inside the system. Results are summarized in a table listing fundamental principles of leak detection. Architectural fundamentals for technology-based leak detection systems (LDSs) are explored and the chapter concludes by exploring fundamental taxonomy of LDS approaches, including technology-based systems and direct observation by humans.