This chapter reviews commonly used analytical methods specified by the US Environmental Protection Agency and present the concept of detection limit which has profound impact on whether the pollutant is detected or not; that is, polluted or not polluted. It analysis of volatile organic compounds is used as an example to discuss this subject, followed by the definitions and uses of various detection limits in environmental applications. The chapter gives assurance and quality control procedures used by the analytical laboratory. It introduces the concept of detection limit and how to use the analytical knowledge for data interpretation. The chapter adequately demonstrates how important it is to understand the concept of detection limit and how the detection limit should be set to serve the purposes of site assessment and remediation. It is recommended for environmental professionals to understand the analytical method prior to evaluating the analytical results.