In today's competitive and highly litigious world, it is critical that any laboratory generating data for the environmental and allied industries have a world-class Quality Assurance Program. This Plan must conform to the requirements of every agency and client with whom the lab does business. The goal of such a program is data defensibility; i.e., data validity. Data (usually qualitative analyte (compound or element) identifications and quantitative numerical results) are the end results of nearly all analytical laboratory processes, and the source of revenue. Clients pay for results. The clients expect the results to be accurate, precise, and repeatable. If their data has to go to court, the laboratory will be called upon to defend the accuracy and precision of their work. Without a strong QA program, this will be impossible. The potential implications and repercussions of non-defensible lab data are far-reaching and very costly in terms of loss of future revenues and in legal judgments.